Comments by Pentagon chief Jim Mattis come before UN Security Council discusses gas attack in Syrian rebel-held town.

The United States is not ruling out the use of military force in Syria in response to a recent poison gas attack in a rebel-held town, James Mattis, the US defence secretary, has said.

His comments on Monday came hours before the United Nations Security Council was due to discuss (19:00 GMT) Saturday’s deadly attack in Douma amid growing international outrage.

“The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons,” Mattis said before the start of a meeting at the Pentagon with Qatari Emir Sheikh Tamim bin Hamad Al Thani.

“Working with our allies and our partners from NATO to Qatar and elsewhere, we are going to address this issue,” said Mattis.

Russia is considered the guarantor of a 2013 agreement to rid Syria of its chemical weapons stockpile and prevent its ally, the government of President Bashar al-Assad, from using them.

When asked if he could rule out taking action such as launching air raids against al-Assad’s forces, Mattis replied: “I don’t rule out anything right now.”

Responding to US remarks, Russian President Vladimir Putin warned against speculation and “provocation” during a phone conversation with German Chancellor Angela Merkel on Monday, the Kremlin said.

“The leaders exchanged opinions on the situation in Syria including the accusations against Damascus by a number of Western countries of using chemical weapons. The Russian side stressed the unacceptability of provocation and speculation on this matter,” the Kremlin said.

US PresidentTrump condemned the attack and blamed Russian President Vladimir Putin and Iran for backing al-Assad

“Many dead, including women and children, in mindless CHEMICAL attack in Syria,” Trump wrote on Twitter on Sunday.

In April last year, the US president ordered air raids on Syrian government facilities in the wake of a chemical attack in Khan Sheikhoun , a rebel-held town, which killed at least 80 people.

Syria airbase attacked

Both Moscow and Damascus have denied carrying out a chemical attack in Douma, calling the allegations “fabrications”.

Mattis’ remarks also came after missiles struck a Syrian airbase in Homs province earlier on Monday, state media reported, with Russia and Syria blaming Israel for carrying out the attack.

Two Israeli fighter jets, using Lebanese airspace, fired eight missiles at the T-4 military airbase, the Russian military said, but offered no further information.

The attack at the airbase, located 40km west of Palmyra, killed and wounded several people, Syrian state news agency SANA reported, citing an unnamed military source.

Israel has not responded to the accusations.