Lucas Jackson/Reuters (Lucas Jackson)

Russia blocked a U.S.-written resolution at the United Nations Security Council on Thursday that would have extended an international investigation into chemical weapons use in Syria, angering diplomats who said Moscow was making it difficult to prevent future attacks.

The Russian veto is the 10th that Moscow has made to protect its ally, the government of Syrian President Bashar al-Assad. It means that the Joint Investigative Mechanism (JIM), a body formed to learn who was responsible for chemical weapons attacks on civilians in Syria, expires at midnight Thursday. The U.S. proposal sought to extend it for another year.

The Security Council vote became another flash point in the deteriorating U.S.-Russia relationship, as representatives from Washington and Moscow each blamed the other for causing the investigatory panel to die. The vote came just a few hours after President Trump urged the renewal of the JIM, tweeting that it was needed “to ensure that Assad Regime does not commit mass murder with chemical weapons ever again.”

Nikki Haley, the U.S. ambassador to the United Nations, said Russia had struck a “deep blow” to efforts to find the responsible parties and hold them to account.

Haley acknowledged that other fact-finding bodies could become vehicles for investigating who is to blame, such as the International Commission of Inquiry on Syria. Then she issued a stark warning to Damascus.

“If it is necessary, there is the United States of America,” she added. “The Assad regime should be on clear notice. The United States will not accept Syria’s use of chemical weapons. As we did in April, we will do it again if we must. It would be wise for the Assad regime to heed this warning.”

In April, after a chemical weapons attack on Syrian civilians, the United States launched dozens of Tomahawk cruise missiles at a military airfield from where the planes bearing the weapons took off.

The JIM is a panel created by the United Nations and the Organization for the Prohibition of Chemical Weapons to monitor whether Syria is upholding a treaty that bans their use. It has blamed both the Syrian government and militants of the Islamic State for using chemical weapons in the war that grew out of protests in 2011.

But Russia has bristled at a report last month that said the Syrian military had used sarin gas during an April 4 attack on the village of Khan Sheikhoun that killed more than 90 people and provoked Washington to target the air base a few days later. The report also blamed Islamic State militants for a mustard gas attack at Um Hosh in Aleppo in September 2016.

To investigate the sarin attack it blamed on the Syrian military, the JIM investigators looked at photos, conducted interviews and analyzed soil samples. But they never visited the village where the attack occurred.

Russia called the work “amateurish” and charged the investigators’ reports are routinely biased against the Syrian government.

Vassily Nebenzia, Russia’s ambassador to the U.N., said Russia supported the idea of renewing the mandate but insisted that flaws be corrected. He proposed a draft purporting to do that, but it did not get enough votes to pass.

“There is nothing balanced about the U.S. draft resolution,” he said, charging that none of the flaws were addressed. “You bear the responsibility if the mechanism cannot be salvaged.”

But ambassadors of the United States and its allies were scathing in criticizing Russia.

“Russia has killed the Joint Investigative Mechanism,” Haley said.

“What a shame it is that Russia has brought us to this point,” she added. “What a shame it is that Russia has revealed itself to have allegiances to the Syrian regime, not to the truth or the protection of innocent victims.”