Iran fired six ballistic missiles and 29 drones at Saudi Arabia’s Prince Sultan air base in an attack Friday that injured at least 15 troops, including five seriously, according to two people briefed on the matter. U.S. officials initially reported that at least 10 U.S. troops were injured, including two who were seriously wounded.

More American forces are reaching the Middle East, with a Navy ship carrying about 2,500 Marines having now arrived in the region, U.S. Central Command announced Saturday. The USS Tripoli, an amphibious assault ship, as well as the elements from the 31st Marine Expeditionary Unit that are aboard, are based in Japan. They were conducting exercises in the area around Taiwan when the order came to deploy to the Middle East almost two weeks ago.

Central Command said that in addition to the Marines, the Tripoli also brings transport and strike fighter aircraft, as well as amphibious assault assets to the region. The USS Boxer and two other ships, along with another Marine Expeditionary Unit, have also been ordered to the region from San Diego.

Before the arrival of the Marines, the U.S. military had already built up the largest American force in the region in more than 20 years, including two aircraft carriers, several other warships and some 50,000 troops. The USS Gerald R Ford, the nation’s newest aircraft carrier, recently left the Middle East for repairs and supplies in Europe after a fire in a laundry room that affected some of the ship’s sleeping quarters.

Secretary of State Marco Rubio said Friday the United States can meet its objectives “without any ground troops.” But he also said Trump “has to be prepared for multiple contingencies” and that American forces are available “to give the president maximum optionality and maximum, opportunity to adjust to contingencies should they emerge.”

The Saudi base had come under come attack twice earlier in week, including an incident that injured 14 U.S. troops, according to the people, who were not authorized to discuss the matter publicly and spoke on the condition of anonymity. In the other attack, no one was injured but a U.S. aircraft was damaged.

The base, which is about 96 kilometers (60 miles) from the Saudi capital of Riyadh, is run by the Royal Saudi Air Force, but also used by U.S. troops. The installation has been targeted almost since the beginning of the war, which on Saturday reached the one-month mark.

Army Sgt. Benjamin N. Pennington, 26, was wounded during a March 1 attack on the base and died days later. He is one of the 13 service members who have been killed in the war.

The Pentagon did not immediately respond to an email seeking comment Saturday regarding the American casualties at the Saudi base.

Central Command said Friday that more than 300 service members have been wounded in the war. Most have returned to duty, while 30 remained out of action and 10 were considered seriously wounded.

Iran has responded to attacks by the United States and Israel with strikes against Israel and neighboring Gulf Arab states. The war has upended global air travel, disrupted oil exports and caused fuel prices to soar. Iran’s stranglehold on the Strait of Hormuz, a strategic waterway, has exacerbated the economic fallout.

With the economic repercussions extending far beyond the Middle East, President Donald Trump is under growing pressure to end Iran’s chokehold on the strait. The latest attacks on the Saudi air base happened after Trump claimed talks on ending the war were going “very well.”

Trump said he had given Tehran until April 6 to reopen the strait. Iran says it has not engaged in any negotiations.