U.S. President Joe Biden said Washington had sent a private message to Tehran about Yemen-based, Iranian-backed Huthi rebels responsible for attacks on international shipping in the Red Sea.
"We delivered it privately and we're confident we're well-prepared," Biden told reporters at the White House on January 14 hours after U.S. and other forces conducted a second strike against Huthi sites in Yemen.
Details about the private message were not disclosed. The United States and Iran do not have formal diplomatic relations, and affairs between the two nations have generally been handled through the Swiss Embassy in Tehran since 1979.
The Huthi rebels -- whose attacks against Israeli-linked and other commercial vessels in the Red Sea have prompted military responses from the United States and its allies against Huthi targets in Yemen -- have pledged "strong" retaliation following a second strike on January 13.
The threat amplifies concerns of major military confrontations stemming from Israel's war against Hamas, the U.S.- and EU-designated terrorist group, in Gaza, which like the Huthi movement enjoys Iranian support.
SEE ALSO: 'Separate' They Stand: Despite Iran's Support, Huthi Rebels' Independence Gives Tehran CoverRussia also voiced accusations at the United Nations that the Western moves risk a dangerous escalation.
"This new strike will have a firm, strong, and effective response," Al-Jazeera quoted Huthi spokesman Nasruldeen Amer as saying on January 13.
He claimed there had been no casualties, no injuries, and no "material damages" from the second wave of strikes.
The threat came after the U.S. military carried out its second strike in two days, this time against what U.S. officials said was a radar site.
A day earlier, the United States and the United Kingdom conducted dozens of air strikes against Huthi targets in Yemen. The Huthis said those attacks killed five of their fighters.
The Huthis have denied they are trying to interfere with international shipping but say they are targeting Israeli-linked ships in support of Palestinians amid Israel's ongoing war against Hamas.
But many of the vessels they have targeted have had no clear connection to Israel.
The United States has said it is not seeking a conflict in the region but has warned the Huthis that it will not tolerate further violent disruption to a crucial international shipping lane.
The White House said on January 10 that the Huthi attacks were "unlawful and escalatory."
Reuters quoted another Huthi spokesman, Mohammad Abdulsalam, as saying on January 13 that a strike overnight that struck a military base in Sanaa had no major effect on the group's capacity to block Israeli-linked ships from using the route.
Moscow has condemned the United States' and its allies' Red Sea response.
WATCH: Who are the Huthi rebels, why did U.S. and U.K. forces strike now, and what are the rebels' links to Iran? Hannah Porter is a Yemen expert and senior research fellow with the Ark social enterprise group.
Your browser doesn’t support HTML5
Who Are The Huthi Rebels And What Are Their Links To Iran?
Russia's envoy to the United Nations, Vasily Nebenzia, told a meeting of the Security Council late on January 12 that those countries' actions were "personally" escalating the Gaza conflict and encouraging a spread of Hamas support in the region.
The U.S. Treasury Department announced new sanctions on January 12 on two companies in Hong Kong and the United Arab Emirates as part of a crackdown on the financial network funding the Iranian-backed Huthi rebels in Yemen.
“The United States continues to take action against the illicit Iranian financial networks that fund the Huthis and facilitate their attacks,” Treasury Undersecretary Brian Nelson said in the statement.
The Huthi group, which controls Sanaa and much of the west and north of Yemen, has also fired drones and missiles up the Red Sea at Israel itself.