ADEN (Reuters) - Unidentified gunmen shot dead a colonel in Yemen’s southern resistance in Aden on Tuesday night, a local official said, the latest in a string of assassinations in the city often carried out by Islamist militants.

The gunmen opened fire on a car containing resistance leader Jalal al-Awbali in the Dar Saad district of northern Aden, killing him immediately, the official said.

Islamist militants from both al Qaeda in the Arabian Peninsula (AQAP) and the Yemeni wing of the Islamic State group have staged attacks throughout southern parts of Yemen, including in Aden, for years.

The rate of attacks in Aden has accelerated since July, when local forces backed by the government of President Abd-Rabbu Mansour Hadi and a Saudi-led military alliance recaptured the city from the Houthi militia after months of street fighting.

Insecurity in Aden, the biggest prize yet won by Hadi in Yemen’s nine-month civil war, threatens to undermine the campaign waged on his behalf against the Houthis and army units loyal to former president Ali Abdullah Saleh.

In a sign of growing international concern over Yemen’s security problems, a U.S. drone strike killed four suspected AQAP militants in central Yemen on Tuesday, the first such attack since September.

Western nations have been quietly increasing pressure on Saudi Arabia to seek a political deal to end the conflict, U.N. diplomats have said. The country’s warring parties have accused each other of violations of a ceasefire that began on Dec. 15.

(The story has been corrected to clairfy that the gunmen shot dead one person who was a colonel and a resistance leader, not two people)