The U.N. Security Council voted unanimously Wednesday to authorize an increase in the African Union force in Somalia from 12,000 to about 17,700 and expand its areas of operation in an effort to intensify pressure on al-Shabab militants who recently joined al-Qaida.

As part of its strategy to weaken al-Shabab, the council also ordered a ban on the export and import of charcoal from Somalia, calling the fuel "a significant revenue source" for the militant group.

The council adopted the resolution on the eve of a conference on Somalia on Thursday in London, where senior representatives from more than 40 governments and international organizations are expected to adopt a new approach to the country's myriad problems.

"This is an important resolution, an important building block toward tomorrow's conference," Britain's U.N. Ambassador Mark Lyall Grant said.

He said the British-sponsored resolution gives the AU force, known as AMISOM, the troops and resources necessary to capitalize on the gains it made in pushing al-Shabab fighters out of the capital, Mogadishu, and to increase the military pressure.

U.S. Ambassador Susan Rice said the additional resources for AMISOM "could make a decisive difference in weakening terrorism and bringing peace" to Somalia.

Somalia has had transitional administrations for the past seven years but has not had a functioning central government since 1991, when warlords overthrew a longtime dictator and turned on each other, plunging the impoverished Horn of Africa nation into chaos. The weak transitional government has been fighting against al-Shabab, which began as a movement to oust Ethiopian troops from Somalia some six years ago.

The resolution authorizes an increase in AMISOM's troop ceiling to 17,731 and puts Kenyan troops, which are trying to advance on al-Shabab's southern stronghold of Kismayo, under the force's umbrella.

It expands AMISOM's operations to the south, including the Juba area, and to Baidoa, a major base for al-Shabab in south central Somalia which Ethiopian and Somali forces are trying to retake. Diplomats said Ethiopia did not want to put its troops under AMISOM.

The resolution also expands the U.N.'s logistical and equipment support package for AMISOM, including for the first time funds for nine utility helicopters and three attack helicopters.

Diplomats, speaking on condition of anonymity because negotiations were private, said some European nations and the U.S. initially balked at the cost _ estimated at about $300 million _ and wanted assurances that the money, which will come from the U.N.'s regular budget, will produce results.

The European Union currently pays the salaries of AMISOM troops. A senior EU official, who could not be named under EU rules, said earlier this week that the 27-member regional group is willing to help pay for more AU troops in an effort to clear al-Shabab militants from the country.

Unlike U.N. peacekeeping forces, AMISOM has been carrying out offensive operations.

For the first time, the Security Council authorized AMISOM "to take all necessary measures as appropriate ... to reduce the threat posed by al-Shabab" in its areas of operation, in conjunction with Somali security forces.

Lyall Grant said that expanding AMISOM to areas beyond Mogadishu "will increase the pressure on al-Shabab, further decreasing the space available to them."

One of the unresolved political issues is the follow-on to Somalia's transitional government.

The resolution states for the first time that the mandate for the transition will expire on Aug. 20 and declares that any extension "would be untenable." It calls on the Somali parties to agree on "inclusive and representative post-transitional arrangements."

Rice, the U.S. ambassador, stressed that "the time for political progress is now."

She expressed regret that the council didn't include support for maritime assets for AMISOM, which the U.S. believes would be valuable in its efforts to improve security. "We hope this council will revisit this discussion in the coming months," she said.

Philippe Bolopion, U.N. director for Human Rights Watch, criticized the council for failing to strengthen "the bare-bones human rights monitoring capacity of the U.N. in Somalia."

"While the Security Council has limited influence to stop al-Shabab abuses, it is doing a grave disservice to Somalia's long-suffering civilians by not fully addressing AMISOM's violations," he said.