WASHINGTON -- U.S. forces launched a series of airstrikes in Somalia over the weekend -- one against Islamic State of Iraq and Syria (ISIS) militants and four against al-Shabab. A total of 40 militants were killed -- 36 al-Shabab militants and four with ISIS, CBS News national security correspondent David Martin reports.

ISIS' growing presence in Somalia could become a "significant threat" if it attracts fighters fleeing collapsing strongholds in Syria and Iraq, experts tell The Associated Press, and already it seems to be influencing local al-Shabab extremists to adopt tactics like beheadings.

The U.S. military this month carried out its first drone strikes against ISIS fighters in Somalia, raising questions about the strength of the group that emerged just two years ago.

ISIS burst into public view in Somalia late last year as dozens of armed men seized the port town of Qandala in the northern Puntland region, calling it the seat of the "Islamic Caliphate in Somalia." They beheaded a number of civilians, causing more than 20,000 residents to flee, and held the town for weeks until they were forced out by Somali troops, backed by U.S. military advisers.

Get Breaking News Delivered to Your Inbox Click here to view related media. click to expand

Rise and fall of ISIS in Syria and Iraq

Since then, ISIS fighters have stormed a hotel popular with government officials in Puntland's commercial hub of Bossaso and claimed their first suicide attack at a Bossaso security checkpoint.

This long-fractured Horn of Africa nation with its weak central government already struggles to combat al-Shabab, an ally of al-Qaida, which is blamed for last month's truck bombing in the capital, Mogadishu, that killed more than 350 in the country's deadliest attack.

The Trump administration early this year approved expanded military operations in Somalia as it puts counterterrorism at the top of its Africa agenda. The U.S. military on Sunday told The Associated Press it had carried out 26 airstrikes this year against al-Shabab and now ISIS.

For more than a decade, al-Shabab has sought a Somalia ruled by Islamic Shariah law. Two years ago, some of its fighters began to split away to join ISIS. Some small pro-ISIS cells have been reported in al-Shabab's southern Somalia stronghold, but the most prominent one and the target of U.S. airstrikes is in the north in Puntland, a hotbed of arms smuggling and a short sail from Yemen.

The ISIS fighters in Puntland are now thought to number around 200, according to a U.N. report released this month by experts monitoring sanctions on Somalia. The experts traveled to the region and interviewed several imprisoned ISIS extremists.