(Bloomberg) -- Secretary of State Antony Blinken warned that the US would respond “decisively” if Iran or its proxies attack Americans, the sternest warning yet as the Biden administration tries to keep Tehran from joining the war between Israel and Hamas.

“The United States does not seek conflict with Iran,” Blinken told a UN Security Council meeting Tuesday. “We do not want this war to widen. But if Iran or its proxies attack US personnel anywhere, make no mistake: We will defend our people, we will defend our security, swiftly and decisively.”

Blinken also urged others on the 15-member Security Council, including Russia and China, to tell Iran not to open another front against Israel or attack its partners, and to hold them accountable if it does so.

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“To all the members of this council: If you, like the United States, want to prevent this conflict from spreading, tell Iran, tell its proxies — in public, in private, through every means — do not open another front against Israel in this conflict. Do not attack Israel’s partners.”

Blinken’s remarks were the latest in a stepped-up US messaging campaign to call out what it says is Iran’s involvement in violence in the region against US forces, along with its support of Hezbollah fighters in Lebanon who have launched rocket attacks on Israel in recent days. On Monday, the US said it would hold Iran responsible for a spate of drone and rocket attacks on US forces in the region.

In a phone call with France’s President Emmanuel Macron on Oct. 15, Iran’s President Ebrahim Raisi warned that the conflict could expand if Israel doesn’t stop what he called its crimes, “including the killing of the people and the siege of Gaza,” according to the semi-official Tasnim news agency.

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It’s a complicated move for the Biden administration, which is flowing more forces into the Middle East, including two aircraft carrier groups, additional fighter jets and missile-defense systems. The US says the moves are aimed at making adversaries such as Iran think twice before joining the conflict, but they have also provoked fears that the US will bring about the outcome it wants to avoid by expanding its presence in a volatile situation.

Hamas fighters killed about 1,400 people and seized more than 200 hostages with an Oct. 7 raid on Israel, and Israeli forces have responded with airstrikes that have killed more than 5,000 people, according to Palestinian authorities. 

Israel has vowed to wipe out Hamas — which is designated a terrorist organization by the US and the European Union — permanently through a ground campaign in Gaza. But the US, its allies and adversaries have all expressed concern that such a military move could serve as the trigger for others to enter the conflict.

In his remarks Tuesday, Blinken also called for restraint. He said Israel has the right to defend itself, but “the way it does so matters.”

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