By the BBC's Frank Gardner

The hunt for members of al-Qaeda has spread to the tribal heartland of Yemen.

In the rugged, lunar landscape east of the capital, a gun battle raged through the morning, as Yemeni troops fought with armed tribesmen.

The local tribesmen were believed to have been sheltering two, or possibly three, leading Islamist militants. They are suspected of belonging to Osama Bin Laden's network.

As the two sides traded fire, killing a dozen people, at least one of the militants is reported to have got away.

Significant

The numbers involved here may be small, but this is a significant step in the war on terror.

President Saleh had talks in Washington

For the first time since 11 September, an Arab government has used military force to confront suspected members of Bin Laden's network.

It is no coincidence that Yemen's President, Ali Abdullah Saleh, recently held talks with President George W Bush.

Amid rumours of Yemen being next on Washington's target list after Afghanistan, the Yemeni Government is clearly keen to co-operate with the West.

Phase two

Although there were no initial reports of United States troops being involved in the operation, this may well mark the start of what some would see as phase two in the war on terrorism.

The Yemeni authorities have a sizeable task ahead of them.

In a country where the number of guns outnumbers the population by three-to-one, the government has little control outside the main cities.

Although the number of active supporters of Osama Bin Laden is thought to be small, Washington is convinced that al-Qaeda cells are continuing to operate in the country.

In October last year suicide bombers blew a hole in a US warship in Aden harbour, killing 17 sailors.

Yemen roots

Suspicion immediately fell on Osama Bin Laden and, since then, the Yemeni authorities have been working closely with the US Federal Bureau of Investigation to arrest and interrogate suspects.

Washington has been working for some time now to uncover the extent of support for Osama Bin Laden in Yemen.

Bin Laden's family come from Yemen

It is known that Yemenis have been amongst his fighters in Afghanistan, alongside Saudis, Egyptians and other Arabs.

It is also known that some Yemeni Islamists sympathise with al-Qaeda's extreme view that confrontation is inevitable between the Islamic world and the West because of what it sees as the injustices meted out by the West to Muslim states such as Iraq.

Osama Bin Laden's own family originated in the Hadramaut area of Yemen, moving to Saudi Arabia in the last century.

Yemenis were amongst those who joined him in Afghanistan in the 1980s to confront the Soviets, and then returned home, later to become bored and disillusioned.

The problem for both the US and Yemeni investigators is distinguishing between heavily armed tribesmen who are also devout Muslims, and those who may or may not be prepared to carry out armed action on behalf of al-Qaeda.