U.S. Special Envoy for Yemen Tim Lenderking is visiting Saudi Arabia and Bahrain to coordinate on regional security and concerns over Iran, and for talks on U.N.-led peace efforts for Yemen, the State Department said on Tuesday.

It said Lenderking would also discuss the detention by Yemen's Iran-aligned Houthi movement, the de facto authority in the north, of some Yemeni staff at the U.S. embassy compound in the capital, Sanaa, which was closed in 2015.

The State Department had earlier said the majority of the local employees had been released, without saying how many had been detained or when. The U.S. embassy in Sanaa shut after the Houthis ousted the internationally recognised government from Sanaa in late 2014. It has since operated out of Riyadh.

Mohamed Ali al-Houthi, head of the Houthi supreme revolutionary committee, in a Twitter post on Sunday criticised the United States for "abandoning" local staff, but did not directly refer to their detention.

Lenderking has visited the region frequently this year as part of U.N.-led efforts to engineer a ceasefire in Yemen needed to restart political talks to end the war, in which a Saudi-led coalition intervened in March 2015 against the Houthis.

The conflict, which has killed tens of thousands of people and caused a dire humanitarian crisis, is seen as a proxy war between Saudi Arabia and Iran.

Washington has been pressing Riyadh to lift a coalition blockade on Houthi-held ports, a condition from the group to start truce talks. Riyadh wants a simultaneous deal.

Lenderking's visit comes as Houthi forces advance in areas south of the main port of Hodeidah following a withdrawal by coalition forces in the area. The movement has also recently made territorial gains in gas-rich Marib, the government's last northern stronghold, and in oil-rich Shabwa in the south.