How serious is Somali Shabab threat to attack US?

Al Qaeda's franchise in Somalia threatened to attack the US in its territory, how capable the group is to launch such attack?

Somalia's rebal group Al Shabaab El Mogahdin, rebels are seen as lacking the numbers and unity of purpose to carry out a threat by one of their leaders to attack the United States on its own territory.
Fuad Mohamed Khalaf, number three in the group, said on Monday the al Qaeda-linked Islamists would carry out attacks in the United States if President Barack Obama and American citizens did not convert to Islam.
While al Shabaab, managed to carry out a major bomb attack in Uganda earlier this year, Somalia analysts based in the country and outside this week dismissed the threat against Washington as propaganda. "I do not believe al Shabaab can implement its threat to the U.S. It is more propaganda," Rage Farah, a history lecturer at a Mogadishu university, told Reuters.
"Al Shabaab itself admits that it failed to overcome AU (African Union) forces. They may threaten Uganda, countries that are within their reach."
Together with soldiers loyal to the Somali government, the AMISOM African Union peacekeeping force has been making gains over al Shabaab fighters in Mogadishu in recent weeks, suggesting the insurgents are falling back in their campaign to overthrow the government.
Abdiwahab Abdi Samad, a lecturer in armed conflict and peace studies at the University of Nairobi, said the group has no more than 3,000 to 5,000 local fighters, who have been joined by about 200 foreign recruits from countries such as Afghanistan and Pakistan.
The insurgents are also split down the middle in terms of ideology, with half espousing jihadist tendencies and the other half just interested in bringing Somalia under their rule, Samad added.
Al Shabaab, which means "the youth" in Arabic, controls most of the south of Somalia and much of Mogadishu, from where they launch attacks on the United Nations-backed government.
Headed by President Sheikh Sharif Ahmed, the Transitional Federal Government is protected by the 8,000-strong African Union force, mainly drawn from Uganda and Burundi.
In July, two suicide bombings in the Ugandan capital killed 79 people. Al Shabaab claimed responsibility and identified the presence of Ugandan troops in Somalia as grounds for further reprisals.
Security officials in East Africa have stepped up their state of alert following renewed threats from al Shabaab. A bag containing a hand grenade, that was about to be put on a Kampala-bound bus in Nairobi, exploded last week, killing two people.
In his comments to reporters this week, Khalaf also urged Islamist fighters in Yemen -- seen in the West as an emerging source of radicalised militants -- to join in the fight against Christians.
Afyare Elmi, a Somali professor of politics at Qatar University in Doha, said al Shabaab's "loud claims" were intended to show the group's fighters in a favourable light.
But Rashid Abdi, a Nairobi-based Somalia analyst for the International Crisis Group, said that while there was an element of propaganda in the group's latest threat, it should not be dismissed completely.
"They are at war with the whole of the West because it is behind the Transitional Federal Government ... the threats shouldn't be taken lightly," Abdi said.