Aden (AFP) - Yemeni pro-government forces and rebels completed an exchange of hundreds of prisoners Thursday, an official said, amid a shaky ceasefire on the third day of UN-sponsored peace talks in Switzerland.

"We have successfully completed the process of exchanging the prisoners," said Mokhtar al-Rabbash, a member of the prisoners' affairs committee, which is close to the government.

The swap involved 370 Huthi rebels and 285 pro-government fighters.

It took place in the Yafaa district of the southern province of Lahj, along the border with the central Bayda province, witnesses said.

The swap was slowed down by concerns over security along the route linking the two exchange points, Rabbash said.

The International Committee of the Red Cross in Sanaa, which was involved in a previous swap, said earlier that the organisation was "not aware of such an exchange".

Little information has emerged from the open-ended talks in Switzerland aimed at ending Yemen's devastating conflict.

"The talks are still ongoing and progressing," UN media advisor Charbel Raji said without elaborating.

Delegates said they had been asked not to talk to the media.

A fragile ceasefire declared by forces loyal to embattled President Abedrabbo Mansour Hadi, began on Tuesday. It remained shaky, with reports of sporadic violations on the ground.

The Saudi-led coalition supporting Hadi accused Huthi Shiite rebels of flouting the ceasefire immediately, and acknowledged that it had "responded to these violations".

Heavy clashes erupted overnight in Marib province, east of rebel-held Sanaa, between pro-government Popular Resistance fighters and the insurgents, military sources said.

Pro-Hadi forces stormed a military base that was controlled by rebels in northern Marib, following clashes that left an unspecified number of casualties on both sides, the sources said.

An officer said the rebels "did not adhere to the ceasefire" and used the base to fire at pro-Hadi forces.

The Huthis and allied renegade troops loyal to former president Ali Abdullah Saleh were also accused by residents of bombing areas controlled by pro-Hadi forces in the flashpoint city of Taez.

More than 5,800 people have been killed in the country -- about half of them civilians -- and more than 27,000 wounded since March, according to the UN.