Iran's Rouhani said the battle in Syria would continue until rebels were pushed out of the whole country, especially in Idlib, but he added that any military operations should avoid hurting civilians.
Much of the motivation for the change, officials said, stems from growing doubts about whether Russian Federation, which Trump has said could be a partner, is able and willing to help eject Iran.
Russian President Vladimir Putin has arrived in Tehran, where he is to meet Iranian and Turkish leaders.
"The new policy is we're no longer pulling out by the end of the year", said James Jeffrey, a retired senior Foreign Service officer who last month was named Secretary of State Mike Pompeo's "representative for Syria engagement". Although the USA has about 2,000 troops and outposts in Syria, President Donald Trump has said he wants to pull those forces out after the war against the Islamic State group dislodged the extremists from vast territories it once held there and in Iraq.
"We have to force the United States to leave".
"That means we are not in a hurry", he said. "I say this because the USA is making it clear it is not bluffing this time, and Turkey is similarly against the offensive".
Mr Rouhani said Syria must regain control over all its territory.
But the more activist policies he outlined, and only in vague terms, could increase the likelihood of a direct confrontation with Iran, and potentially with Russian Federation.
Just months after proclaiming to the American public that USA soldiers will completely withdraw from Syria "very soon", President Donald Trump has reportedly agreed to a new strategy under which the more than 2,000 troops now occupying the war-torn nation will remain "indefinitely".
"We consider any assault on Idlib to be a risky escalation of the conflict in Syria", said USA ambassador to the United Nations Nikki Haley Friday in NY.
Pompeo first listed Iran's withdrawal from Syria as one of 12 USA demands of Tehran in a May speech at the Heritage Foundation.
But Hassan Hassan, a Syria expert and a fellow at the Washington-based Tahrir Institute for Middle East Policy, said he is doubtful an offensive is imminent, pointing to Turkey getting US backing in opposing a major offensive.
United Kingdom names and charges two Russian men for nerve agent attack
It's unclear how much further Britain or other countries will go in punishing Russian Federation for the poisoning. May said. "It was nearly certainly also approved outside the GRU at a senior level of the Russian state".
"Assad has no future, but it's not our job to get rid of him", Jeffrey declared.
Speaking after the meeting, Iran's Supreme Leader Ayatollah Ali Khamenei lashed out at the USA and vowed to keep working with Moscow.
The first test of the administration's expanded role in Syria may come sooner rather than later in Idlib, in the northwest part of the country. On paper, the leaders reiterated pledges to seek a negotiated solution to Syria's seven-year conflict, to preserve the country's territorial unity, to eliminate al-Qaeda-linked terrorists and to assure the safe return of millions of displaced Syrians.
Already, almost a half-million people have been killed in the grinding civil war, which began first as a popular uprising against Assad and later devolved into a sectarian and regional conflict.
Russian president Vladimir Putin travelled to Tehran for high-level diplomatic talks with his Iranian and Turkish counterparts today.
"The battle for Idlib is going to be the final major battle", said Hilal Khashan, a political science professor atthe American University of Beirut, told Reuters before the summit. Humanitarian organizations have warned of an unprecedented level of civilian bloodshed, and Trump himself has threatened US retaliation if an all-out offensive is launched, especially with the use of chemical weapons. Abdurrahman said suspected Russian warplanes carried out the attack.
One recent air strike targeted rebel forces, but human rights organizations claim 13 civilians, including children were killed in the attack.
The Assad government was not directly represented at the summit, nor were the United States and other Western powers. Jeffrey added that the attack would be a "reckless escalation".
USA officials have said they will defend themselves if attacked.
"We don't want Idlib to be turned into a bloodbath", he stressed.
In her remarks at the U.N. Security Council, Haley said the USA has been clear with Russian Federation and other nations that "we consider any assault on Idlib to be a unsafe escalation of the conflict in Syria".