Separately, a government official said tests on the remains of five people recovered in northern Yemen this week showed they were not Europeans taken hostage in the north last year.
Yemen's supreme security committee said northern rebels were dragging their heels on implementing a cease-fire deal struck with Sanaa in February to end fighting.
"The (rebels) returned again to some sites after leaving, established new checkpoints, and committed numerous violations and attacks on citizens and some public and private installations," Yemen's state news agency reported the committee as saying.
The rebels were also refusing to hand over land mines removed from the conflict zone which were supposed to be destroyed by the state, the agency said.
On their website, the rebels said military units and local officials had entered unhindered a number of northern areas on Monday, including Malahith, Razih and Al-Zaher.
"These steps come as we confirm that we do not interfere with matters of the local authorities, and that we have never done so and never will," rebels said in a statement posted on Monday.
A series of small explosions hit Yemen's main southern city of Aden late on Monday, prompting a heavy deployment of government troops, a local official told Reuters on Tuesday.
The explosions were likely caused by firebombs, the official said, describing the perpetrators as "saboteurs.” Local media attributed the blasts to grenades or locally made explosives.
There were no reports of deaths or injuries.
North and South Yemen united in 1990, but many in the south — home to most of Yemen's oil industry — complain northerners have seized resources and discriminate against them.
Since the botched plane bombing in December, Yemen has stepped up its fight against Al-Qaeda. The government said it had launched air strikes in recent days against what it said are Al-Qaeda militants in the southern part of the country.
State media said on Tuesday three militants were killed in a Sunday raid on the southern province of Abyan, including Jamil Al-Anbari, identified by police as a local leader of Al-Qaeda.
Opposition media had reported that at least six people had died and 23 were wounded in Sunday's attack.
In Sanaa, a Yemeni government official told Reuters that tests on five bodies recovered in northern Yemen this week showed they were the remains of Somalis, not of Europeans taken hostage in the north last year.
The tests were carried out by Yemeni authorities with the help of foreign experts, the official told Reuters, without elaborating.
A German family of five and a Briton are missing in Yemen, held by kidnappers who the government believes have links to Al-Qaeda. They were among a group of nine foreigners kidnapped in the northern region of Saada last June, of which three women — two Germans and a South Korean — were later found dead.
No group has claimed responsibility for the abduction, which occurred in an area where Shiite rebels have been fighting government troops on and off since 2004. The rebels have denied involvement.
