U.S. airstrikes rocked Yemen’s capital, Sanaa, Sunday, killing at least four and wounding over 20, including women and children, when a residential building was hit. Three more strikes targeted the Al Aswad area west of the city.

The attacks, a response to Houthi assaults on Red Sea shipping in solidarity with Palestinians amid Israel’s Gaza offensive, signal an intensifying U.S. campaign against the Iran-backed group.

Iran’s Supreme Leader Ayatollah Ali Khamenei ordered his military to stay on high alert, warning neighboring countries against allowing U.S. forces to use their land or airspace.

Tehran views such cooperation as direct hostility threatening retaliation. Reuters reports Iran sent messages to Qatar, Turkey, Iraq, UAE, and others, emphasizing that aiding U.S. attacks would make them targets. “Standing with the enemy means inviting war,” an Iranian official declared.

Tensions between the U.S. and Iran, already strained over Tehran’s nuclear program, spiked after President Donald Trump demanded compliance with a nuclear deal or face strikes. Iran rejected direct talks, calling threats unacceptable, though it signaled openness to indirect diplomacy via Oman. Russia urged restraint from all sides.

The U.S. bolstered its military presence around Iran, while Tehran fortified defenses. Foreign Minister Abbas Araghchi dismissed direct U.S. negotiations as futile but affirmed Iran’s willingness for backchannel discussions.

Read more: Israel Launches Massive Airstrikes in Central and Southern Gaza, Killing Palestinians Including Journalist

Meanwhile, the Houthis’ Red Sea campaign, protesting Israel’s Gaza blockade, triggered the latest U.S. reprisals deepening the Middle East’s volatile standoff.