SANAA - The president of Yemen has resigned along with his prime minister in protest at the takeover of the capital Sanaa by Shia Houthi rebels, creating a dangerous political vacuum.
President Abdrabbuh Mansour Hadi said he could not continue after the Houthis failed to honour a peace deal.
Parliament has reportedly refused to accept the resignations.
Security sources have told the BBC that Yemen's intelligence chief, Ali Hassan al-Ahmedi, has also stepped down.
There are reports of explosions in the southern port city of Aden, where President Hadi has a strong following.
Houthi rebel figures publicly welcomed the resignation of the president with one proposing the creation of a ruling council.
The council would include Houthi-led groups, Abu al-Malek Yousef al-Fishi was quoted as saying by Reuters news agency.
Houthi leaders had previously committed themselves to withdrawing from key positions around the presidential palace and the home of President Hadi.
The US, which is helping fight al-Qaeda militants in Yemen, said it was still assessing the implications of President Hadi's move.
Local officials in Aden told Reuters news agency that unidentified gunmen had attacked two military vehicles in the city early on Friday, local time.
Three explosions were heard during the attack, which was followed by the clashes, said one of the officials, who declined to be identified.
Explosions and clashes were also reported by Qatar-based broadcaster al-Jazeera, which said they were happening on the city's Martyrs' Square.
In his letter of resignation, seen by the Associated Press news agency, Mr Hadi said the parties had reached a "deadlock".
"We found out that we are unable to achieve the goal, for which we bear a lot of pain and disappointment," he said.