Students at nearly a dozen U.S. colleges have planned protests in recent weeks targeting their schools' investments in the oil, gas and coal sectors, part of a coordinated push to stop colleges from funding companies that contribute to global warming.

Nearly 700 students and their supporters have already staged sit-ins and held rallies at more than six campuses this month, including University of Denver, Columbia University, New York University and Swarthmore College in Pennsylvania. Protests at Harvard University and University of Massachusetts Amherst resulted in nearly two dozen arrests.

Students at Vassar College in Poughkeepsie, New York, on Monday launched what they said will be a weeklong sit-in just outside the school president's office. More protests at additional schools are planned for the coming weeks, said Lindsay Meiman , a spokeswoman for environmental group 350.org.

Student environmental activists, in loose coordination with Bill McKibben's 350.org -- the group that led the national effort against the Keystone XL pipeline -- for years have been pushing their schools to dump their holdings in the fossil fuel industry. Some 30 schools or their related funds have announced plans to partially or fully shed fossil fuel investments, according to 350.org.

But student activists now have set their sights on one reason why they say more colleges haven't followed suit: perceived conflicts of interest among college endowment officials.

Many college officials, including those responsible for endowment funds, have gotten rich off investments in the fossil fuel industry, activists say. Those investments could be the reason why more schools aren't rushing to drop holdings in oil, gas and coal companies.

"Students are supportive of our cause," said Julia Berkman-Hill, 20, a junior and sociology major at Bowdoin College in Brunswick, Maine. "What's obstructing progress is the [college's] board of trustees."

More than 40 people serve on Bowdoin's board. Student activists have identified three "decision makers" they say "have no incentive to engage productively on divestment when they are themselves financially tied to the fossil fuel industry." One of them, famed investor Stanley Druckenmiller, who is also on the board of the Environmental Defense Fund, dismissed the allegation as nonsense.

Bowdoin won't dump its investments in fossil fuel companies, spokesman Scott Hood said. Student activists tried to persuade the college's board in 2014 but failed. "We applaud our students activism and agree that meaningful steps must be taken to mitigate the dangerous impact of climate change," Hood said.

Bowdoin officials have promised that their school will become "carbon neutral" by 2020.

Like Bowdoin, other colleges, while generally resisting divestment campaigns, have taken other steps to address climate change.

For example, while Yale University's nearly $26 billion endowment fund refuses to completely divest from investments in fossil fuel companies, it announced earlier this month that it had sold close to $10 million in energy-related investments it said were "inconsistent with our principles." Harvard announced in 2008 that by this year its greenhouse gas emissions would be 30 percent below 2006 levels.