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 US government is unlikely to strike back so close to leaving office.

De Mistura 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.

He said that moves by Syria to escalate the military conflict could have tragic consequences for 275,000 civilians still in the eastern part of Aleppo, and drew parallels to an 87-day siege of Vukovar, Croatia by Serbian forces in 1991.

"I am very concerned about what can happen before January 20," de Mistura told a group of Social Democratic lawmakers. "We are very concerned (about the possibility of Assad) ... taking over in a brutal aggressive way what is left of eastern Aleppo," he said. "It could be tragic. It could be a new Vukovar."

De Mistura said little was known about Trump's Middle East policy, but there might be a chance of progress in ending the Syrian war if Trump stuck to his campaign pledge to fight Islamic State together with Russian President Vladimir Putin.

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

Steinmeier cited talks about bringing relief supplies into Aleppo via Turkey, but gave no details and said there were no guarantees for the success of the effort.

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 programme in exchange for sanctions relief could exacerbate tensions in the region.