The US defence secretary says al-Qaeda has "seized the opportunity" in Yemen amid reports that fighters from the group had stormed a border post near Saudi Arabia.
Speaking during a visit to Japan on Wednesday, Ashton Carter said both the Houthis and al-Qaeda in the Arabian Peninsula (AQAP) were taking advantage of disorder following the collapse of Yemen's central government.
Referring to AQAP activity in Yemen, Reuters news agency quoted him as saying: "We see them making direct gains on the ground there as they try to take territory, seize territory in these battle lines ... we are observing that," in an apparent reference to an AQAP attack on Tuesday of a Saudi border post that killed two soldiers.
Carter's comments came as the US said it was supplying intelligence to the Saudi-led coalition bombing of Houthi rebel positions across Yemen, and pledged to expedite arms supplies to the Gulf kingdom.
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While on a visit to the Saudi capital Riyadh on Tuesday, Tony Blinken, deputy secretary of state, said that Saudi Arabia's forceful military campaign was sending a "strong message to the Houthis and their allies that they cannot overrun Yemen by force".
"As part of that effort, we have expedited weapons deliveries, we have increased our intelligence sharing, and we have established a joint coordination planning cell in the Saudi operation centre," Blinken said.
American reiteration of its support for the Saudi-led coalition comes as the United Arab Emirates' foreign minister, Abdullah bin Zayed al-Nahyan, said that the coalition cannot rule out any options during the military intervention in Yemen, including sending ground troops to fight the Houthi rebels.
Meanwhile, Iran's President Hassan Rouhani has called for a political solution to the crisis, saying that "war and bloodshed must stop in this area immediately and a complete ceasefire must be established and the strikes must stop" during a joint press conference with visiting Turkish President Recep Tayyip Erdogan.
Rouhani said he hoped the two countries, "with the help of other countries in the region" would contribute to "peace, stability, a broader government and dialogue" between Yemenis.
Rouhani's comments came even as Iran dispatched a naval destroyer and another vessel to the Gulf of Aden on the Red Sea on Wednesday, though Iran's English-language state broadcaster Press TV quoted Rear Admiral Habibollah Sayyari as saying that the ships would be part of an anti-piracy campaign "safeguarding naval routes for vessels in the region", rather than it being directly related to the conflict in Yemen.
Saudi Arabia has accused Iran of arming the Houthi rebels in Yemen, though Tehran denies the allegations.
'Catastrophic' situation in Aden
The Houthi fighters swept into Sanaa in September and have since tried to expand their control across the country.
In February, they placed President Abd-Rabbu Mansour Hadi under house arrest before he fled to his power base in the southern city of Aden and then to Saudi Arabia.
Two weeks ago, Saudi Arabia, along with a coalition mainly consisting of other Gulf countries, launched a military campaign consisting mainly of air strikes targeting areas across Yemen, particularly those where the Houthis are strongest.
Since then, the Houthis launched a ground offensive, capturing much of the city of Aden, one of the last remaining strongholds of Hadi supporters, while Saudi-led air strikes have heavily bombarded the coastal city.
The International Committee of the Red Cross (ICRC) has warned of a "catastrophic" situation in Aden, as it reported that it has flown medical personnel for the first time into the country, amid delays that have worsened the humanitarian situation.
More than a 100,000 people have fled their homes after the Saudi-led coalition launched air strikes, according to UNICEF, the UN agency responsible for children welfare.
Source: Al Jazeera and agencies