Would a Russian veto at the UN trigger US strikes?

Russia has used its veto power 11 times to block action against its Syrian ally, shielding President Bashar al-Assad's government from sanctions, war crimes investigations and an inquiry into chemical weapons attacks.

Speculation about a 12th Russian veto on Syria is again rife after the United States on Monday put forward a draft resolution on a 30-day ceasefire in Eastern Ghouta and Damascus.

The move was in response to the failure of a UN-approved humanitarian ceasefire to take hold despite backing from Russia, which is helping Syria's offensive in Eastern Ghouta, a rebel enclave on the outskirts of the capital.

The operation has reportedly killed more than 1,100 civilians since it was launched in mid-February.

A separate US-drafted measure to set up an inquiry into chemical weapons attacks is under negotiation, but diplomats said talks with Russia had hit a wall.

With two measures on the table, US Ambassador Nikki Haley has warned that the United States is ready to act in Syria "if we must" to address the use of chemical weapons and "inhuman suffering."

"When the international community consistently fails to act, there are times when states are compelled to take their own action," Haley said on Monday.

"We also warn any nation that is determined to impose its will through chemical attacks and inhuman suffering, most especially the outlaw Syrian regime: the United States remains prepared to act if we must."

It remains unclear when the United States will put its draft resolutions up for a vote at the council, which is increasingly frustrated by its inability to change the course of the war in Syria.

Deja vu

Haley's threat was a repeat of the warning delivered to the council in April last year, just before President Donald Trump ordered missile strikes on a Syrian air base over the sarin gas attack on the opposition-held village of Khan Sheikhun.

UN diplomats are openly talking about "deja vu" as the United States again signals that it is considering military action in Syria.

The threats are ratcheting up pressure on Russia.

"The Russians are feeling the heat. They are afraid that the Americans might re-engage in Syria in a more robust way," a council diplomat said on condition of anonymity.

Another council diplomat pointed to the anniversary of the Khan Sheikhun attack on April 4 as a possible timeframe for US action.

"The Trump administration is resorting to a naming-and-shaming strategy on Russia at the Security Council," said Nicholas Heras, Syria expert at the Washington-based Center for a New American Security.

"This is the beginning of a larger discussion with Russia about what will happen with other areas in the country," in particular the southwest where a de-escalation zone has been set up by the United States, Jordan and Russia, said Heras.

"What the Trump team is doing now is initial posturing: It's saying: 'What you are doing in Ghouta, don't do it anywhere else. If you try to do it anywhere else, there will be consequences.'"

Haley's warning drew a strong response from Moscow on Tuesday.

After last year's US strike on the Shayrat air base, Russian Foreign Minister Sergei Lavrov warned that "if a new strike of this kind takes place, the consequences will be very serious."

Russia's head of the general staff Valery Gerasimov warned of "retaliatory measures" if any Russian serviceman is hit in a US strike.

Russia has military staff on the ground helping the Syrian offensive in Eastern Ghouta, which lies near the capital of Damascus.