A pair of US F-15E aircraft fly over Iraq after conducting air strikes in Syria in 2014, shown in this US Air Force picture
Washington (AFP) - US President Donald Trump and his French counterpart Emmanuel Macron agreed Tuesday on a joint response in the event of another chemical attack in Syria, as Washington warned Damascus would pay a "heavy price" for such a move.
The tough language, in a phone call between the two leaders, came a day after the White House accused President Bashar al-Assad's regime of preparing a potential chemical weapons attack.
Washington's warning to Damascus -- which prompted criticism from regime allies Russia and Iran -- coincided with a wave of US-led coalition air strikes that killed nearly 60 people at a Syrian prison run by the Islamic State group.
The coalition has been striking IS in Syria and Iraq since mid-2014 but has also been involved in recent confrontations with Assad's forces, raising fears of the United States being drawn into Syria's civil war.
A Pentagon spokesman said Tuesday that US intelligence had noticed suspect activity at the launch site of an apparent chemical strike by the regime two months ago.
The April attack on the rebel-held town of Khan Sheikhun was reported to have killed at least 87 people, including many children, and images of the dead and of suffering victims provoked global outrage.
Washington launched a retaliatory cruise missile strike days later against the Shayrat airbase where it said the attack was launched -- the first direct US action against the regime, which denies any use of chemical weapons.
White House spokesman Sean Spicer warned Monday night that if "Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."
Trump and Macron agreed Tuesday on "the need to work on a joint response in the event of a chemical attacks in Syria."
After a meeting last month with Assad's ally, Russian President Vladimir Putin, Macron had drawn a "very clear red line" on the use of chemical weapons in Syria and warned of reprisals.
- 'Refuse to get drawn in' -
America's warning to Damascus drew condemnation both from the Kremlin, which said it considered "such threats against the Syrian leadership to be unacceptable," and from Tehran.
"Another dangerous US escalation in Syria on fake pretext will only serve ISIS, precisely when it's being wiped out by Iraqi and Syrian people," Foreign Minister Mohammad Javad Zarif tweeted.
Coalition forces on the ground have accused pro-regime fighters of targeting them in recent weeks, as they shot down two Iran-made attack drones and a Syrian fighter jet.
Defense Secretary Jim Mattis has highlighted the importance of maintaining communication with Russia, which is backing Assad's forces with air strikes.
The Pentagon chief insists the United States is not taking sides in the Syrian conflict, saying late Monday that US forces would not fire on targets "unless they are the enemy, unless they are ISIS."
"We just refuse to get drawn into a fight there in the Syria civil war," he said.
Monday's coalition strikes hit an IS-run jail in Syria's Mayadeen at dawn, according to the Syrian Observatory for Human Rights, a Britain-based war monitor which said they killed 42 prisoners and 15 jihadists in Mayadeen, a large town in the eastern province of Deir Ezzor.
Pentagon spokesman Adrian Rankine-Galloway confirmed coalition strikes on Mayadeen on Sunday and Monday, targeting IS "command and control facilities" and other "infrastructure".
The allegations of casualties at the prison "will be provided to our civilian casualty team for assessment," he added.
- Russian brothers -
Most of Deir Ezzor province is controlled by the jihadists and it has been the target of air strikes by both the coalition and the Syrian army and its Russian ally.
The jihadists, who seized control of large parts of Syria and Iraq three years ago, are under pressure in both countries.
US-backed forces are pushing to oust IS, also known as ISIS and ISIL, from its last major urban strongholds, Raqa in Syria and Mosul in Iraq.
The Russian intervention marked a major turning point in the conflict and pro-regime forces have since made significant gains, including their recapture of second city Aleppo.
Assad on Tuesday toured an airbase in western Syria set up for the Russian intervention, inspecting equipment and even climbing into the cockpit of a Sukhoi fighter jet.
"The Syrian people will not forget the support of their Russian brothers," Assad wrote in the visitors' book at the Hmeimim base.