Jeffrey said the Syrian-regime under Bashar al-Assad, along with the support of the Iranians and the Russian airforce, are responsible for the country's destruction.

The comments directly clash with Russian president Vladimir Putin, who in August launched a diplomatic offensive to get the Europeans help pay for reconstruction.

The EU says that they are willing to look into the issue of reconstruction when the regime transition in Syria is formally underway.

The US comments also follow a four-nation summit with the leaders of Turkey, Russia, France and Germany in Istanbul over the weekend to find a ceasefire in Syria.

The United States, which was not invited by the host Turkey, has since sent Jeffrey on a 10-day diplomatic tour of Europe and the Middle East to drum up support for peace in Syria.

The seven-year conflict has killed hundreds of thousands, fuelled radicalisation, and led to an exodus of refugees into Turkey, Lebanon, Jordan - and Europe, notably Germany.

The presence of military forces from Turkey, the United States, Russia, Israel and Iran has only heightened tensions.

"This is basically large military forces of important countries facing off against each other in some ways," warned Jeffrey.

The Istanbul summit demanded a lasting ceasefire in Idlib, a city in northwestern Syria, ravaged by war.

It also called for a committee to draft Syria's post-war constitution before the end of the year.

Asked if the United States still had a meaningful role in Syria, Jeffrey noted Washington's diplomatic weight and the some 2,000 US troops fighting the Islamic state, a militant Islamist group, in the war-torn country.

"If you really think that in an area where we have thousands of troops, and our diplomatic weight, that we are not playing a role in this, I am sorry, that is not how foreign relations work," he said.

The US, along with France and the UK, had launched airstrikes earlier this year against chemical weapons facilities in Syria.

The European Union at the time backed the strikes.

Turkey, under a separate deal struck in September, had also agreed to create a 15 to 20km-wide demilitarised zone around the insurgent-held Idlib region.

The September deal was struck in Sochi, Russia, between Turkey's president Recep Tayyip Erdogan and Putin.

Both the Russian and Iranian government support Assad, whose regime has dropped barrel bombs and launched chemical weapon attacks against its own people.