GENEVA (Reuters) - Iran shares the United Nations’ concern about a potential humanitarian catastrophe in Syria’s Idlib province and will seek to avert it, Hossein Jaberi Ansari, special assistant to Iran’s foreign minister, told reporters on Tuesday.

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Iran and Russia are President Bashar al-Assad’s main backers in Syria’s seven-year-old civil war. Last week, Russian and Syrian warplanes resumed a bombing campaign in Idlib, the last rebel enclave in Syria, after weeks of quiet, in an apparent prelude to a full-scale offensive.

“We are also worried. We are going to work toward that not happening,” Ansari said as he arrived in Geneva for U.N. talks about setting up a Syrian constitutional committee. The talks are hosted by U.N. envoy Staffan de Mistura and also include senior Russian and Turkish officials.

Idlib is the insurgents’ only remaining major stronghold and a government offensive could be the war’s last decisive battle.

Turkish President Tayyip Erdogan said in an article in the Wall Street Journal that a Syrian government offensive in Idlib would cause humanitarian and security risks for Turkey, Europe and beyond.

Erdogan failed to secure a pledge for a ceasefire from Russia and Iran at a trilateral summit in Tehran.

U.S. President Donald Trump’s national security adviser said on Monday the United States, Britain and France had agreed that another use of chemical weapons by the Syrian government would result in a “much stronger response” compared to previous Western air strikes there.

U.S. officials have said in recent days they have evidence that Syrian government forces are preparing chemical weapons ahead of a planned assault on Idlib.

“I am not going to tell the world ahead of time what we are going to do, it is just not my style,” U.S. Defence Secretary Mattis told reporters in Washington on Tuesday.

Trump has twice ordered U.S.-led strikes against targets in Syria in response to what Washington called the Assad government’s use of chemical weapons against civilians.

“The first time around (Assad) lost 17 percent of his pointy nosed air force airplanes. He’s been warned and so we’ll see if he’s wised up,” Mattis added.