Tens of thousands of Yemeni civilians are trapped by fighting during a US-backed army offensive on Islamist militants and urgently need help, the International Committee of the Red Cross (ICRC) said on Wednesday.

The Yemeni army is trying to recapture towns in the southern province of Abyan that were seized by al Qaeda-linked militants last year during a popular uprising against President Ali Abdullah Saleh, who formally relinquished power in February.

In support of the army campaign, the United States has stepped up drone strikes against suspected members of an al Qaeda branch that is one of its main global security concerns.

"We are extremely concerned about the people trapped inside, and about the dire situation in Jaar, Shaqra and in nearby areas where fighting is going on," Eric Marclay, the head of the ICRC delegation in Yemen, said in a statement.

"Our staff were there a few days ago to assess the situation and found serious, urgent needs that, if not met, could lead to the displacement of over 100,000 people. Thousands of people have already fled to safer places."

Residents are facing food, power and water shortages, while and health-care services are inadequate, the ICRC said.

On Tuesday, all the roads to Abyan were blocked and movement in and out of the province was restricted, it added.

The Geneva-based ICRC urged all combatants to grant it immediate access and security guarantees.

Concerned about the humanitarian and security crisis in Yemen, Gulf Arab states and the West pledged more than $4 billion in aid to the impoverished state last month, $3.25 billion of which pledged by Saudi Arabia alone.

About 40 percent of Yemenis live on less than $2 a day. Aid agencies said in May almost half of them lack enough to eat.

In northern Yemen, tribal mediators have begun trying to end months of fighting between Shi'ite rebels and Sunni Muslim Salafis near the Saudi border, a Salafi spokesman said.

The "Houthi" rebels, who have fought the government on and off since 2004, exploited the anti-Saleh uprising to escape state control in the rugged northern province of Saada, where they have also battled their ultra-orthodox Salafi rivals.

Saudi Arabia briefly intervened on the Sanaa government side in 2009. The world's leading oil exporter is hostile to the rising regional power of Shi'ite Iran and faces periodic unrest among its own Shi'ite minority in eastern provinces.

The Houthis, now in control of much of the north, agreed on Tuesday to join a national dialogue aimed at resolving Yemen's multiple political conflicts.