UNITED NATIONS, Dec. 20 (Xinhua) -- Outgoing UN Special Envoy for Syria Staffan de Mistura on Thursday was unwilling to comment on U.S. President Donald Trump's decision to withdraw troops from Syria.

"It is totally premature for making any comment on my side on the implications, the effects of that important decision, which was announced yesterday -- both on the political ground and on the military ground," de Mistura told reporters after his briefing to the Security Council.

"I am unable to make any comment. It's premature for me to do so."

De Mistura, after more than four years as UN special envoy for Syria, steps down by the end of the year. He will be replaced by Geir Pedersen of Norway.

The White House said Wednesday that the Trump administration has started returning U.S. troops home from Syria after claiming victory in the fight against the Islamic State terrorist group in the region.

Asked earlier Thursday to comment on the U.S. decision, French ambassador to the United Nations Francois Delattre said: "We need more than ever America's political commitment to a political solution in Syria."

De Mistura said the United Nations will never abandon Syrians.

Asked to leave a message for the Syrian people as he steps down as special envoy, de Mistura said: "I have been holding the fort for four and half years. I have always said: I was not ever going to abandon them, and what I meant is the UN, will never abandon them. They should know that, they have gone through terrible things."

There are only two constants in the Syrian war, he said. "One, there has never been a constant moment -- look what has been happening, we get constant changes both militarily, politically -- going through the four years. And the second constant: the Syrian people have been constantly suffering in between that."

He said he would probably write a book on his experiences in Syria.

Earlier on Thursday, in his briefing to the Security Council, de Mistura expressed optimism about the prospects of a political solution in Syria.

"Ultimately the question will be -- now is the time to look at it even more so: will the peace be won? We believe it should and it can. And will it be sustainable? We believe it can and it should. That is why a real political process is required," he said.

A list for the constitutional committee is almost complete. Its rules of procedure have become clearer, he said. "But we need to go that extra mile (before the establishment of the constitutional committee)."

The United Nations has reservations about the 50 names put forward by the foreign ministers of Iran, Russia and Turkey -- guarantors of the Astana process for Syria -- as civil society representatives for the committee.

De Mistura's own list was rejected by the Syrian government.

Under an intra-Syrian agreement reached at Sochi, Russia, in January 2018, the constitutional committee, tasked for a constitutional reform process, is to be composed of representatives of the Syrian government, the opposition and civil society representatives -- each with 50 members. There was no objection to the other two lists.