A soldier was killed and another two injured when gunmen attacked a police car in an attempt to release a convict in downtown Yemen's capital Sanaa on Saturday, security sources told the Yemen Post.

The gunmen, who were onboard two pickups, fired at and pursued the central prison truck to release, Saleh Al-Shatawai, who had been convicted to death, they explained.

"The incident took place early Saturday while the convict was leaving Althawra hospital where he was checked under the order of the prosecution," they said, but gave no more details including the charges for which Al-Shatawai has been convicted.

The guards of the police truck resisted the attack, with one of them killed and two injured as a result.

Separately, the security authorities defused an explosive device that was placed inside the car of a senior security officer in Hadramout province, state media reported.

The explosive device was discovered immediately after the personal driver arrived to take Colonel Salim Ali Al-Sufra from his workplace, said Fahmi Al-Saiari, director of the Hadramout security office.

"The Colonel noticed the bomb under his car, which appeared as a strange stuff, and then called the authorities," he said, adding experts were sent immediately to defuse it.

Investigations have been launched into the two incidents, which came amid alarming security disorder and acts of sabotage in key parts of Yemen.

Tens of senior military and security officers including intelligence agents have been killed in main cities including the capital Sanaa so far this year.

Meanwhile, the security situation is casting a cloud on the political transition at a time when Yemen is gearing up for a comprehensive national dialogue conference.

The dialogue conference is expected to be held in coming months under the West-backed power-transfer deal which was reached after the 2011 unrest.