French warplanes turned the fury of their bombing runs to central Mali on Monday after a column of Islamist guerrillas swept southward along the Mauritanian border and captured the town of Diabaly, in what was described as fierce fighting with Malian troops.

The new push brought the desert fighters to within 250 miles northeast of Bamako, the capital. It also dramatized the extent to which the irregular Islamist forces, well armed and mobile aboard speedy pickup trucks, remain a threat even after four days of French bombing and the deployment of more than 500 French troops to bolster the overwhelmed Malian army.

French President Francois Hollande’s government has vowed to pursue the buildup of French ground and air forces and remain in Mali as long as necessary to hold off the Islamist militias, introduce a pan-African force and train the disorganized Malian army to restore state authority across the vast West African nation.

The 15-nation Economic Community of West African States (ECOWAS) pledged to contribute more than 3,000 troops for the pan-African force, which France has been eager to get organized so it can depict its intervention as a cooperative effort. ECOWAS military chiefs set a meeting for Tuesday, but the arrival of the first African soldiers, promised since Saturday, has repeatedly been delayed.

“The African forces are gathering,” said Gen. Shehu Abdulkadir, a Nigerian who is to lead what will be called the International Support Mission for Mali, or MISMA by its French-language initials.

French military intervention in Mali

Britain offered to provide several U.S.-made C-17 transport planes to ferry in the African troops and their equipment.

The Pentagon may become involved in the military operations by providing airlift and “limited logistical support” to French troops fighting in Mali, Defense Secretary Leon E. Panetta said Monday.

U.S. defense officials said they were reviewing requests for assistance from France. Islamist fighters and Tuareg rebels have gained control of the northern half of the country over the past year, enabling al-Qaeda’s affiliate in North Africa to function unimpeded in a swath of territory the size of Texas.

“We have a responsibility to go after al-Qaeda wherever they are,” Panetta told reporters as he began a week-long trip to Europe. “We’re going after them in Yemen and Somalia, and we have a responsibility to make sure that al-Qaeda does not establish a base for operations in North Africa, in Mali.”

Panetta declined to provide details about what kind of military assistance the Pentagon might bring to the conflict but said one option under consideration would be to deploy transport aircraft to move French troops or equipment.

The Obama administration previously ruled out placing “U.S. boots on the ground” in Mali. Officials traveling with Panetta declined to comment when asked whether U.S. transport aircraft might land in Mali to help the French, or whether the territory remained off-limits.

State Department spokeswoman Victoria Nuland said some provision of communications gear or other help appeared likely.

Planning before last week’s emergency French intervention called for African forces to deploy in the fall, and ECOWAS leaders were caught short by Hollande’s decision to telescope events, according to Tony Chafer, a specialist on French-African relations at the University of Portsmouth in England. It will take some time to organize, train and transport any pan-African force beyond a few symbolic units, he said, “and they will need somebody to provide airlift capacity.”

The United States, France, the U.N. Security Council and several African countries have been working for months on a joint plan to intervene militarily in Mali, one of the poorest and most remote countries in the world.

The planning, however, has been undermined by strategic disagreements, a lack of firm commitments to send troops and Mali’s internal political dysfunctions. The country’s democratically elected president was toppled in March in a coup led by a rogue army captain who had received military training in the United States. Factionalism has worsened since then as Islamist fighters tightened their grip on the north.

Another complication is that the United States is prohibited by law from providing direct military assistance to the Malian government because of the coup. The Pentagon had to shut down training and aid programs in Mali last year and remove virtually all military personnel.

The fighting, meanwhile, is driving tens of thousands of Malians out of their villages. The United Nations said Monday that an estimated 30,000 people may have been displaced as a result of the latest fighting. Additionally, a U.N. spokesman said, some reports indicate that even more civilians are trying to flee the north but are being prevented from doing so by Islamist groups.

Since March, 230,000 people have been displaced by the fighting and insecurity, according to Eduardo del Buey, a U.N. spokesman.

French Defense Minister Jean-Yves Le Drian said the Islamist offensive that began Wednesday comprised two columns heading south from the vast redoubt in northern Mali, where Islamist groups have roamed freely for eight months and set up a “caliphate” under strict Islamic law.

One column attacked the town of Konna on Thursday and headed toward Mopti, 300 miles northeast of Bamako. It was made up primarily of combatants from the Movement for Unity and Jihad in West Africa, or MUJAO by its French-language initials.

MUJAO is a spinoff of the main North African Islamist group al-Qaeda in the Islamic Maghreb,
or AQIM, which includes many ­battle-hardened Algerians who fought the government for years before migrating to Mali.

Although it had seized Konna, the first column was scattered by French Gazelle helicopter gunships and Mirage 2000D fighter-bombers over the weekend. Bombers pursued retreating fighters northeastward, hitting their rear headquarters in the city of Gao on Sunday and inflicting what residents said were dozens of casualties.

A MUJAO leader, Omar Ould Hamaha, vowed that despite the setback, the group would live to fight another day. He urged the French army to send ground troops into the northern area and fight “like real men.”

“We’ll welcome them with open arms,” he said in a telephone interview with French radio Europe 1. “France has opened the gates of hell. It has fallen into a trap much more dangerous than Iraq, Afghanistan or Somalia.”

The second column, dominated by AQIM fighters, moved farther to the west, along the Mauritanian border. It stormed down Sunday night, and early Monday it took over Diabaly, Le Drian said. But he said the Malian army was putting up stiff resistance with backing from French air power.

Whitlock reported from Lisbon. Colum Lynch at the United Nations and Anne Gearan in Washington contributed to this report.