Secretary of Defense Mark Esper said Monday that some US troops will temporarily remain in Syria in part to protect oil fields in the country from ISIS as other US troops withdraw from northern Syria.

Speaking at a press conference in Afghanistan, Esper said the US withdrawal from northern Syria will take “weeks not days” and that troops temporarily staying near oil fields are not in the current phase of withdrawal of troops from northern Syria.

“Our forces will remain in the towns that are located near to the oil fields,” Esper said. “A purpose of those forces working with the (Kurdish-led Syrian Democratic Forces) is to deny access to those oil fields by ISIS and others who may benefit from their revenues earned.”

A US official told CNN up to several hundred troops may stay in eastern Syria to protect the oil fields and continue the fight against ISIS. The US military is “providing options and contingency plans to give (President Donald Trump) options,” the official said, although orders “have not been given yet.”

On Twitter, Trump has repeatedly mentioned securing oil in Syria in recent days.

The US military has long had military advisers embedded with the Syrian Democratic Forces near the Syrian oil fields at Deir Ezzoir ever since the area was captured from ISIS. The loss of those oil fields denied ISIS a major source of revenue, a one-time source of funds that has differentiated the organization from other terror groups.

The oil fields are assets that have also been long sought after by Russia and the Assad regime, which is strapped for cash after years of civil war. Both Moscow and Damascus hope to use oil revenues to help rebuild western Syria and solidify the regime’s hold.

In a bid to seize the oil fields, Russian mercenaries attacked the areas, leading to a clash that saw dozens if not hundreds of Russian mercenaries killed in US airstrikes, an episode that Trump has touted as proof he is tough on Russia. That action helped deter Russian or regime forces from making similar bids for the oil fields.

The US forces near the oil fields remain in place and senior military officials had previously told CNN that they would likely be among the last to leave Syria.

The secretary said there have been discussions about keeping US troops in northeastern Syria for a longer period, but there has not yet been a military option along those lines presented to Trump.

On Sunday, The New York Times reported that a senior administration official said the President is leaning toward approving a new Pentagon plan to keep a small contingent of American troops in eastern Syria, perhaps 200 special operations forces.

If approved, the plan would walk back Trump’s order to the Pentagon to withdraw nearly all of the 1,000 US troops from northern Syria. Trump’s announcement about pulling out troops prompted a barrage of criticism from within his own party, including Senate Majority Leader Mitch McConnell, Sen. Lindsey Graham and former US Ambassador to the United Nations Nikki Haley.

The troops would be kept to combat ISIS and block the advance of Syrian government and Russian forces into the region’s oil fields, the official told the Times.

Since last week, Trump has been considering a plan that would be for counter-terrorism efforts and would leave a couple of hundred troops in northeast Syria, near the border with Iraq, a senior administration official told the Times. The official said the President favors this concept.

The senior administration official told the Times it was highly likely US troops would be kept along the Iraqi border area, away from the ceasefire zone that Vice President Mike Pence negotiated with Turkish President Recep Tayyip Erdogan last week. The main goal would be to prevent ISIS from resurging in Syria and Iraq.

Another benefit of the proposal would be helping the Kurds keep control of oil fields in the east, the official said.

The proposal is being discussed among top American policymakers and commanders, according to the Times, which cited three other administration and Defense Department officials who spoke on the condition of anonymity to discuss confidential military planning.

CNN has reached out to the White House for comment.

In December, Trump discarded advice from allies and officials and ordered staff to execute the “full” and “rapid” withdrawal of US military from Syria, in a sharp reversal from previously stated US policy. Former Secretary of Defense James Mattis, who resigned in December, said in a recent interview with CBS News that he reached his breaking point with the Trump administration after the President decided to withdraw US troops from war-torn Syria.