The United States is reported to be escalating its largely secret war against al-Qaida in the turbulent Horn of Africa state of Somalia amid signs the al-Shabaab Islamist network remains a danger despite recent setbacks.

The insurgents, who suffered a series of military defeats in 2011-12 in their battle against a shaky Western-backed transitional federal government installed in late 2006, have been written off several times before and regrouped -- just as they appear to be doing now.

In 2012, after al-Shabaab had been driven out of Mogadishu, Somalia's war-battered capital, the southern port of Kismayu, its financial center, and other major towns by African Union forces supported by the United States, France and Britain, the U.S. administration of President Barack Obama was proclaiming Somalia a major success in the global war against al-Qaida.

The Americans provided satellite and drone intelligence to the African forces led by Kenya, Uganda and Ethiopia, as well as arms supplies during their year-long offensive that ultimately drove the Islamist fighters into the countryside.

The campaign, the U.S. journal Foreign Policy reported, "appeared to be a signature achievement of the Obama administration: backing an African-led effort to dent an al-Qaida affiliated insurgency a strategic toehold in the heart of East Africa."

Much of the largely clandestine U.S. operation was run from Djibouti, a former French colony where the Americans maintain a 2,000-strong counter-terrorism force equipped with at least eight MQ-1 Predator unmanned aerial vehicles and eight Air Force F-15E fighters at Camp Lemonnier, a former Foreign Legion outpost on the Indian Ocean.

Now, the U.S. Journal Foreign Policy reports, the Obama administration, which during its first term greatly expanded covert U.S. operations against al-Qaida and its allies across the globe, is providing arms and other assistance for the government of recently elected President Hassan Sheik Mohamud and the intelligence services of Western-leaning breakaway states Puntland and Somaliland, which are suitably rewarded for assisting.

This violates a 1992 arms embargo on Somalia imposed by the United Nations' Security Council. That was eased in March allow the rearming of the transitional government's ramshackle forces, but Mogadishu is required to inform the Security Council about foreign military assistance and apparently hasn't.

The United Nations' Somalia Eritrea Monitoring Group said in a report released Tuesday that two U.S. air charter companies, believed to be fronts for the United States' Central Intelligence Agency, have increased their clandestine flights to Mogadishu and Puntland in recent months.

The report identified the U.S. firms as the Prescott Support Co. of Florida, and RAM Air Services headquarters in Atlanta.

It said between late 2012 up to March 13 they flew scores of flights, mainly from Djibouti, to Mogadishu to support the National Intelligence Security Agency, and to Bosaso and Galkayo in Puntland to support the Puntland Intelligence Service, which has cooperated with U.S. counter-terrorism operations for more than a decade.

It listed a Prescott Lockheed L-100-30 Hercules and a RAM Saab 340B, both U.S.-registered. The Monitoring Group said it had not been notified of any of the flights.

These flights are only part of the covert U.S. operation under way in East Africa to combat al-Shabaab and al-Qaida in the Arabian Peninsula, based in Yemen across the Gulf of Aden from Somalia.

On April 19, 2011, U.S. Special Forces captured a senior al-Shabaab commander, Ahmed Abdulkadir Warsame, a Somali, aboard a fishing boat en route to Somalia from Yemen. He was held incommunicado for two months' intense interrogation aboard the USS Boxer, an amphibious assault ship, in the Red Sea, before being flown to New York to be indicted in July 2011.

It's not clear to what extent intelligence gleaned from Warsame has aided U.S. efforts to cripple al-Shabaab and al-Qaida in the Arabian Peninsula. But recent U.S. moves to bolster regional forces would indicate a resurgence by the Somali Islamists is anticipated.

Despite internal rifts al-Shabaab is reported to have been reorganizing, recruiting heavily and conducting intense infiltration of the "highest levels" of the transitional government, while carrying out morale-sapping suicide bombings.

Professor Kenneth Menkhaus of Davidson College in North Carolina, an expert on Somalia, said al-Shabaab "remains a strong and dangerous force, capable of extortion, intimidation and assassination.

"This fits the shift of al-Shabaab from what had been a standing army, capable of controlling large swaths of territory, to a decentralized, clandestine terrorist network," Menkhaus said.