SANAA - An al-Qaida linked group has claimed responsibility for Thursday's attack on Yemen's defence ministry that killed at least 52 people, the country's worst militant assault in 18 months."As part of the policy of targeting the operation rooms of pilotless planes, the mujahideen (holy fighters) have heavily struck one of these rooms in the Ministry of Defense headquarters," Ansar al-Sharia (Partisans of Islamic Law) said in a Twitter message posted early on Friday.The group is an offshoot of al-Qaida in the Arabian Peninsula. "Such joint military locations which participate with the Americans in their war against this Muslim nation are a legitimate target for our operations," another tweet said.Thursday's attack, in which 167 people were wounded, was carried out by a suicide bomber and gunmen wearing army uniforms. Some of the dead were foreign medics, including German, Vietnamese, Indian, Filipino nationals.The US military raised its alert status in the region after the coordinated strikes on Yemen, which is also home to what Washington has called the most active arm of al-Qaida.The security threat is an international concern. The impoverished country shares a long border with Saudi Arabia, the world's top oil exporter, and sits close to key shipping lanes.Violence is common in Yemen, where an interim government is fighting southern secessionists and northern Houthi rebels in addition to al-Qaida linked militants, who are seeking to overthrow the government and impose their version of Islamic law. The country is also facing severe economic problems inherited from former president Ali Abdullah Saleh who was forced out of office by a popular uprising in 2011.Islamist insurgents took advantage of the chaos triggered by Saleh's overthrow to seize several southern cities, but were driven out in 2012 in a government offensive aided by US drone strikes.Al-Qaida militants have since killed hundreds of Yemeni soldiers and members of the security forces in a series of attacks, particularly in the southern provinces of the country.In July last year, a suicide bomber wearing a Yemeni army uniform killed more than 90 people rehearsing for a military parade in the capital. Al-Qaida later claimed responsibility.