Russia blocks US-drafted UN resolution to renew international inquiry into who is to blame for chemical weapons attacks in Syria.
It was the 10th time that Moscow has used its veto power to stop council action that targeted its ally Assad regime.
The Joint Investigative Mechanism has to cease operations as its current mandate expires at midnight Thursday.
The UN mechanism determined in late October that forces loyal to Syrian President Bashar al-Assad were at fault for an April 4 chemical weapons attack in northwestern Syria. Its mandate is set to expire Friday.
The U.S. draft text needed nine votes in favor to pass, and received 11. But it also required none of the five permanent members of the council to issue a veto, which Russia did.
Russia's UN ambassador, Vasily Nebenzya, slammed the text, calling it unbalanced and unrealistic.
Russia has been Assad's principal backer in the world body, and has supported him militarily in Syria with an extensive air campaign and ground forces.
"By eliminating our ability to identify the attackers, Russia has undermined our ability to deter future attacks," Nikki Haley, the U.S.'s UN envoy said in a terse statement. "The message to anyone listening is clear: in effect, Russia accepts the use of chemical weapons in Syria."
Dozens of people died in the early April sarin gas attack on Khan Sheikhoun, and repeated accusations of chemical weapons use date back to 2013.
Russia withdrew its own draft resolution after it failed to circumvent council rules to have it considered before the U.S. version. After the vote on the U.S. text Bolivia, a Russian ally, sought a vote on Moscow's proposal. It failed with only four votes in favor.
Resolutions need nine votes to get through the council.
Later, Japan circulated its own draft resolution asking for the investigative panel to be extended for one month, but it is unclear when that vote will take place.
U.S. President Donald Trump pressed earlier for the extension of the mandate, saying on Twitter he needs "all" on the council to renew the so-called JIM "to ensure that Assad Regime does not commit mass murder with chemical weapons ever again."
Russia has been seeking changes in the way the JIM operates. Syria accepted a Russian proposal in 2013 to relinquish its chemical weapons stockpile.