WASHINGTON -- A new report blasting Iran for its involvement in the Syrian civil war is drawing attention in the nation's capital just as U.S. officials sit down with Iranians in Vienna in a last-ditch effort to reach a deal on Tehrans nuclear program.

The report, titled "Iran in Syria," was released earlier this month by a group of Iranian, Syrian and Lebanese activists known as Naame Shaam. It relies heavily on information available in the public domain, particularly Syrian, Iranian and Western media coverage of the Syrian conflict.

It details, in roughly 100 pages, Irans deep-rooted influence in the region, and suggests that Iran, a longtime backer of Syrian President Bashar Assad, is an "occupying force" in Syria that would be able to retain control there using Shiite militias even if Assad falls.

The report also accuses Tehran of permitting the rise of the Islamic State and an al-Qaeda affiliate called Jabhat al-Nusra, the Sunni extremist groups in Syria that a U.S.-led coalition is now targeting. And it says Tehran was complicit in war crimes committed in Syria -- including last year's chemical weapons attack, which drew international attention and for which the U.S. blamed Assad .

More bad news about Iran is hardly what the administration needs at present. With most indications suggesting that the negotiations on Tehran's nuclear program will have to be extended past the Nov. 24 deadline, the White House will need to convince the public and a Congress eager to impose more sanctions on the Iranian government that pursuing the diplomatic route with Tehran remains worth it.

Staffers for the Senate Foreign Relations Committee, which would be responsible for the legislation imposing those new sanctions, were briefed on the report earlier this week, according to sources with knowledge of the briefing.

That Iran holds influence in Syria isnt necessarily breaking news-- I know all about that, Sen. John McCain (R-Ariz.) said Wednesday -- but the documents could become a useful tool for bolstering skepticism among policymakers and the public towards Iran. Lawmakers and staff who are subject to classification boundaries could use the publicly available data and analysis in the report to help make a convincing case for a harder stance towards Tehran.

Fouad Hamdan, the campaign manager for the activists and the executive director of the Netherlands-based Rule of Law Foundation, the foundation backing the group, told The Huffington Post his goal was not to give Republican Iran skeptics further ammunition against a Democratic administration trying to reach a deal with Iran.

Instead, he said, he wanted to give the U.S. a better sense of the "price of relaxing sanctions" as part of a deal. Hamdan added that he would be skeptical of the deal reportedly under consideration , which would guarantee at least one year of breakout time, or one year of foreknowledge before Iran could build a nuclear weapon.

"You cannot trust them from the past," Hamdan said. His group includes a number of Iranian dissidents who were involved in the Green Movement protests against the country's repressive theocratic rule.

Activists like him, he added, want the West to show Iran that "you cannot take over the region." Supportive of Obama but confused about the president's seeming failure to recognize that Iran's vision of the region is hardly one amenable to U.S. interests, Hamdan said his group hopes for "a tough presidency that says enough is enough to Iran."

Hamdan also said he would like negotiators from the U.S. and the other countries currently seeking a deal on Iran's nuclear program -- Britain, France, Germany, Russia and China -- to secure two guarantees during their talks in Vienna. The first, Hamdan said, is that Iran would slowly withdraw from Syria; the second is that Iran would loosen its control in the other nation where it wields major influence and where the U.S. is now fighting the Islamic State: Iraq.

The administration maintains that the nuclear negotiations do not touch on other regional issues. But it has conceded that the fight against the Islamic State has been discussed on the sidelines of the sessions, and Obama reportedly sent Iran's Supreme Leader Ayatollah Ali Khamenei a letter in October explicitly connecting a nuclear deal to the two nations' common interest in fighting the Islamic State.

Administration officials have indicated in recent weeks that in both Iraq and Syria, the U.S. effort against the Islamic State is hamstrung by concerns over Iranian power. In Iraq, officials told HuffPost, the U.S. worries that Iran will turn its Shiite militias on the growing number of U.S. military advisers. In Syria, according to officials quoted in the Wall Street Journal , one reason the U.S. is careful not to strike Assad -- the chief foe of the Syrian moderate rebels Obama plans to train and equip -- is because doing so may harm Iranian forces.

The report's findings raise the stakes for the international community because they lift the Syrian civil war to the level of an international conflict. If verified, the previously unconfirmed level of Iranian strength in Syria is also bad news for the U.S.-backed rebels in the country, who Washington sees as key to both fighting the Islamic State and undermining Assad.

Iran has long backed Assad as a regional proxy. Having him in control in Syria is useful for Tehran for reasons that have little to do with that country: His regime provides Iran with access to another key proxy, Hezbollah, in Lebanon, and therefore the ability to directly threaten Israel. Gen. Qassem Suleimani, the commander of Iran's paramilitary Quds Force, is reportedly leading the Iranian effort in both Iraq and Syria , directing Hezbollah and Shiite militias.

Its no secret that Congress is at odds with the administrations optimism over a potential deal on Irans nuclear program, particularly as the White House suggests it may circumvent skeptical lawmakers in lifting Irans imposed sanctions.

The report comes as the anti-Iran caucus on the Hill grows in both size and volume, and as that group prepares to find new platforms and little resistance from congressional leadership once the upper chamber flips in January.

Lingering on the sidelines of the power dispute is a bipartisan new sanctions bill that would hit Iran's petroleum and mining industries, further weakening an economy already crippled by sanctions. Authored by Senate Foreign Relations Chair Bob Menendez (D-N.J.) and Sen. Mark Kirk (R-Ill.), the bill has hung in limbo since being introduced in the Senate last December. But if and when lawmakers are incensed enough at the White House to move it forward, it will likely find little resistance.

Even in the Democratic-led Senate, the bill found strong backing from 59 co-sponsors -- including several powerful Democrats . The influx of anti-Iran Republicans into the Senate after the midterms and an incoming leadership friendly to that sentiment will only make it more likely that new sanctions legislation will make it through Congress.

When asked this week about his future plans for the bill, Menendez said he would wait and see what happens on the Nov. 24th deadline on the Vienna talks.

But lawmakers from both sides of the aisle have made it incandescently clear in recent days that, should a lackluster deal be reached, theyre ready to push back.

Earlier this week, 43 Republican senators signed on to a letter penned by Sens. Marco Rubio (R-Fla.) and Kirk, warning the White House against avoiding lawmakers over a potential Iran deal.

Supporters of a deal, who say it is the best way to monitor Iran's nuclear activity and to avoid an eventual confrontation between the country and the U.S., have slammed such congressional threats. In a conference call with reporters Friday, Dylan Williams of the pro-Israel advocacy group J Street, which promotes peace in the Middle East, said moves like Rubio's letter are "music to the ears of Iran's hard-liners." Those hard-liners are reportedly worried that a deal and eventual rapprochement with the West would move Iran away from the ideology of the 1979 Islamic Revolution -- and weaken their influence.

The full Iran in Syria report can be read here . Read the previously unpublished policy recommendations associated with it, which Hamdan told HuffPost he is giving to the legislators and analysts he is currently meeting with in D.C.

Ryan Grim contributed reporting to this story.