(Reuters) - A suicide bomber linked to al Qaeda drove a car laden with explosives into a hospital used as a base by Yemen's Shi'ite Muslim Houthi movement on Sunday, killing at least 15 people, and five more died in an ambush in the south of the country.

Militants, tribal and local sources said the first attack took place in the town of Majzar in Maarib province, east of the capital Sanaa, while local officials said the second occurred on Sunday night in the southern al-Bayda province.

Houthi fighters seized Sanaa on Sept. 21 after four days of fighting with soldiers loyal to the Sunni Muslim Islah party. They have since refused to quit the capital, despite an agreement they signed with President Abd-Rabbu Mansour Hadi to make them a part of the government.

Instability in Yemen is a worry for the United States and its Gulf Arab allies because of its position next to Saudi Arabia and shipping lanes which run through the Gulf of Aden. Saudi Foreign Minister Prince Saud al-Faisal told the U.N. General Assembly on Sunday that the situation there posed a threat to international security.

"Dozens of dead and wounded from the rejectionist Houthis in a martyrdom operation by Ansar al-Sharia using a booby-trapped car in Maarib," the militant group, a branch of Al Qaeda in the Arabian Peninsula (AQAP), said in a statement on its Twitter account.

The group said the target was al-Jafra hospital, which had been turned by the Houthis into a base for their operations in the area. Local tribesmen said at least 15 people were killed in the attack and more than 50 were wounded.

In the second attack, local officials said al Qaeda militants ambushed a car used by Houthi fighters, killing five of them. An al Qaeda statement put the Houthi death toll in the attack at six.

There was no immediate word from the Houthi, who are named after the tribe of their leader and founder.

Ansar al-Sharia had previously carried out numerous attacks on military and civilian installations of the U.S.-allied Yemeni government. But the group, which adheres to an austere brand of Sunni Islam which views Shi'ites as heretics, has turned its attention to the Houthis after they captured Sanaa.

Last week, they said they carried out a similar attack on the Houthis in their northern stronghold of Saada province, in which dozens were killed or wounded.

The Houthis captured Sanaa after overrunning an army brigade affiliated to the moderate Islamist Islah party, making them effectively the power brokers in the country.

Their Zaydi Shi'ite sect is related to, but separate from, the sect that rules Iran. They make up 30 percent of Yemen's population of 25 million and ruled a kingdom there for 1,000 years. But they have complained of being marginalized since their last king in Sanaa was overthrown in a 1962 revolution.