The United Nations Special Envoy for Syria has warned that Syrian armed forces could launch a new offensive to crush resistance in eastern Aleppo before Donald Trump formally assumes the U.S. presidency on January 20.

The U.N. Special Envoy for Syria Staffan De Mistura on Tuesday said he was worried that Syrian President Bashar al-Assad could launch a new offensive to crush eastern Aleppo before U.S. President-elect Donald Trump takes office.

Staffan De Mistura did not explain why he thought Syria might make such a move, but European diplomats have said Assad may feel emboldened by Trump's vow to build closer ties with Russia, and the current U.S. government is unlikely to strike back so close to leaving office, according to Reuters.

De Mistura reportedly joined German Foreign Minister Frank-Walter Steinmeier in calling for an end to the bombardment of civilians in Syria and a political solution to the conflict.

Russia views Assad as a bulwark against Islamist militancy, while the Obama administration is pushing for his departure.

Some European diplomats have said Syrian President Bashar al-Assad may feel emboldened by Trump's vow to build closer ties with Russia, and push for a crucial victory in Aleppo.

The city has been devastated, and is the focus of some of the worst fighting of the six-year civil war.

Some 275,000 civilians are still in eastern districts.

Pro-government forces, meanwhile, pushed deeper into Syria on November 23, pressing a week-old offensive that's been bolstered by withering Russian airpower.

Recapturing east Aleppo would be the government's biggest victory yet.

Reuters reports that Rolf Muetzenich, a contender to replace Steinmeier if he is elected German president in February, said Syria and Russia were using the time before Trump's inauguration to step up their attacks on rebel-held areas in Syria.

He said Trump's failure to articulate a clear agenda for the Middle East had created a “conceptual vacuum” that could lead to “further instability and even possible additional proxy wars” in the region, he said.

He also warned that any move by Trump to dismantle the Iran nuclear deal struck under President Barack Obama in which Iran agreed to restrict its nuclear program in exchange for sanctions relief could exacerbate tensions in the region.