BEIRUT (Reuters) – Hezbollah leader Sayyed Hassan Nasrallah said on Sunday that the United States was serving the interests of Islamic State by preventing the Syrian army and its allies advancing in some areas of eastern Syria.

American jets have this year targeted pro-government militias in eastern Syria that were advancing towards Washington’s local allies.

Nasrallah said the United States was delaying the Syrian military in its battle to retake territory from Islamic State.

“The American air force in some areas prevents the Syrian army and its allies from advancing in areas controlled by Islamic State,” said the leader of Hezbollah, a Lebanese Shi‘ite movement designated by Washington as a terrorist group.

“The Americans are working to hinder the battle against Islamic State,” he said in a speech, part of a commemoration for Hezbollah fighters killed in clashes against Islamic State.

His comments came after a senior U.S. official said President Donald Trump was considering putting sanctions on Iran for its support of Hezbollah, and as the U.S. Congress prepares to consider tighter sanctions on Hezbollah itself.

The battlefield situation in Syrian is complex, comprising both a civil war that pits Syrian President Bashar al-Assad’s government against rebels, as well as two separate campaigns against Islamic State.

The Syrian army, backed by Russia, Iran and Shi‘ite militias including Hezbollah, has advanced against Islamic State in an offensive across the central Syrian desert in recent months.

The United States is meanwhile leading a rival international coalition against the Sunni jihadists that includes an alliance of Kurdish and Arab militias.

Russia and the United States have put in place measures to prevent outright conflict between the two offensives against Islamic State, but there have been times when they fired on each other.

Security sources in Lebanon have said that Hezbollah has lost hundreds of fighters in Syria since it entered the conflict in support of Assad in 2013 against rebel groups.

Last week Islamic State counter attacks against the Syrian army and its allies killed a number of Hezbollah fighters including a senior commander, Ali al-Hadi al-Asheq.