BEIRUT • US and allied aircraft will be banned from flying over much of Syria as part of a deal struck by Iran, Russia and Turkey to foster a ceasefire in the Syrian war, a senior Russian diplomat has said.
But a US State Department spokesman later said the agreement, which the United States did not sign, does not "preclude anyone from going after terrorists wherever they may be in Syria".
The spokesman, Mr Edgar Vasquez, said Russian officials' interpretation of their own agreement "makes no sense".
A senior State Department official was at the talks in Astana, Kazakhstan, that led to the deal, which went into effect yesterday.
The agreement aims to establish four "de-escalation zones", where Syrian government and rebel forces are supposed to stop fighting each other. It raised the prospect that after years of government opponents asking the US and its allies for a no-fly zone to protect civilians from the Syrian military's bombings, it could end up being Russia, Syria's ally, that imposes one.
But there are many factors that could undermine the deal, as with previous ceasefires.
It has not been accepted by all opposition groups, and the Syrian government reserved the right to continue fighting what it called terrorist organisations across the country.
The Russian statements could also signal an effort to limit US strikes against Syrian government forces, like the one carried out in retaliation for a chemical attack last month.
They suggested that US warplanes could be barred from all of the most important areas contested by the government and rebels that are not affiliated with the Islamic State in Iraq and Syria (ISIS).
Russian diplomat Aleksandr Lavrentiev suggested that Russian and Turkish warplanes would, like the US-led coalition, be prohibited from flying over the zones.
But Mr Lavrentiev, Russia's special envoy on Syria, seemed to sketch out a broader geographical no-fly zone for US and coalition military planes. He said they would be allowed to fly only in eastern Syria over areas held by ISIS, apparently excluding the entire western spine of the country.
Captain Jeff Davis of the Pentagon would not say if the US military would honour the zones and promise not to fly over them.
No-fly zones have been a contentious issue in the Syrian conflict, now in its seventh year. They have long been requested by rebel groups and rejected by the government.
Disputes about who can fly planes and when - "subtle professional issues", as Russian President Vladimir Putin called them recently - are likely to continue under the new deal.
One of the representatives of the Syrian opposition groups at the talks, Colonel Ahmad Berri, expected to see a full ceasefire in the designated zones.
"The Russians this time are more serious, we sensed it, more than last time," he said.
"The regime will be committed to the deal because the Russians are the guarantor, so if the Russians said no bombing, the regime will stop."
On Friday evening, even before the official start of the ceasefire, families in rebel-held areas that have been routinely bombed went to parks, picnicked and organised anti-government demonstrations.