By Andrew Bolton
Arab regional editor, BBC News
Mediators from the Gulf state of Qatar are in northern Yemen to try to save a ceasefire agreement between the Yemeni government and Shia rebels.
There has been increasing violence in the northern Yemeni province of Saada over the past few weeks. A bomb blast at a mosque on Friday killed 18 people.
The mediators face an uphill struggle as there is a deep lack of trust.
Key issues include the handover of heavy weapons by the rebels and an exchange of prisoners.
The Shia militants, led by Abdulmalik al-Houthi, began their latest rebellion some four years ago.
A ceasefire was agreed last year, but it fell apart in January, and further attempts to keep the peace have since faltered.
The bombing of a mosque used by members of the security forces on Friday has been blamed on the rebels, though they deny any involvement.
That attack followed the killing of a prominent Yemeni member of parliament, Saleh al-Hindi, two weeks ago.
Since then a number of rebels and members of the security forces have died in sporadic clashes.
Most of the people in Yemen are Sunni Muslims, and the government of President Ali Abdullah Saleh accuses the Shia militiamen of trying to overthrow it, and impose religious law.
The rebels accuse the government of corruption and discrimination. But they are particularly angered by the government's current alliance with the United States.
Hundreds of people have been killed since the rebellion began in 2004.
There is a new sense of urgency surrounding the Qatari peace efforts, as the mediators work to try to prevent a return to past levels of violence.