RIYADH (Reuters) - Yemen’s Iran-aligned Houthis launched a projectile into southern Saudi Arabia late on Wednesday which the Saudi-led coalition said landed near a desalination plant without causing damage or casualties.

About 24 hours later, the coalition responded with strikes it said targeted Houthi military sites in northern Hodeidah, including booby-trapped boats the group had prepared for terrorist acts and to threaten international shipping.

The Houthis - who threatened last month to attack 300 vital military targets in Saudi Arabia, the United Arab Emirates and Yemen - announced via their Al Masirah TV that a cruise missile they fired had struck an electricity station in Shuqaiq.

A Saudi-based industry source said the strike targeted a power transformer station near the plant, which led to a small fire that was quickly doused and did not impact operations.

U.S. President Donald Trump was briefed on the reported strike and the White House was monitoring the situation, spokeswoman Sarah Sanders said, amid mounting fears of a military confrontation between Tehran and Washington.

Pentagon spokeswoman Navy Commander Rebecca Rebarich said such attacks were “a significant cause for concern and (put)innocent lives at risk”.

The Yemen conflict is widely seen as a proxy war between Iran and Saudi Arabia, a U.S. ally, though the Houthis deny taking orders from Tehran. The group has stepped up missile and drone attacks on Saudi Arabia in recent months as tensions increased between Iran and the United States.

Concern about a war in the region has grown following attacks on two oil tankers in the Gulf of Oman last week and on four tankers off the United Arab Emirates on May 12, both near the Strait of Hormuz, a major conduit for global oil supplies.

The United States and Saudi Arabia blamed Iran, which has denied responsibility.

Tehran shot down a U.S. surveillance drone which the elite Revolutionary Guards said on Thursday was flying over southern Iran. A U.S. official said it had been shot down in international airspace over the Strait of Hormuz, and Trump said he suspected it had been shot down by mistake.

The Houthis also targeted two Saudi oil pumping stations last month and a Houthi attack last week on Saudi Arabia’s Abha airport wounded 26, Saudi authorities said.

Late on Thursday, the Houthis said they had launched a drone attack against Jizan airport in southern Saudi Arabia, but the Saudi-led coalition spokesman said Saudi air defenses had intercepted it.

The group, which ousted the Saudi-backed, internationally recognized government from power in Yemen’s capital Sanaa in late 2014, stepped up attacks on Saudi Arabia after a lull last year ahead of U.N.-led efforts to end a war that has killed thousands and pushed Yemen to the brink of famine.

To counter Iran’s threats, the U.S. military has sent forces including aircraft carriers, B-52 bombers and troops to the Middle East. But the United States, Iran and Saudi Arabia have all said they do not want a war in the region.

Tension between Iran and the United States has spiked since last year when Trump withdrew from a 2015 deal between Iran and major powers to curb Tehran’s nuclear program and reimposed sanctions on Iran.