Iran has stepped up weapons transfers to the Houthis, the militia fighting the Saudi-backed government in Yemen, U.S., Western and Iranian officials tell Reuters, a development that threatens to prolong and intensify the 19-month-old war.

The increased pace of transfers in recent months, which officials said include missiles and small arms, could exacerbate a security headache for the United States, which last week struck Houthi targets with cruise missiles in retaliation for failed missile attacks on a U.S. Navy destroyer.

Much of the recent smuggling activity has been through Oman, which neighbors Yemen, including via overland routes that take advantage of porous borders between the two countries, the officials said. […]