"The United States is Islam's known enemy and we will never expect mercy from them, nor should they expect mercy from us."
Somali witnesses to Monday's raid said up to six helicopters flew over a village in Barawe district, an area about 250km south of the capital, Mogadishu, before two of them opened fire on the car carrying Nabhan.
Paula Roque, an African security specialist talking to Al Jazeera from Pretoria, said that removing Nabhan was "quite an important step".
"It could weaken any organisational links that are being set up between al-Qaeda and al-Shabab, even though these are yet to be confirmed, but they do surely have ideological links.
"Nabhan had involvement with al-Qaeda as far back as 1998. He could have been implicated in several terrorist attacks that were taken forward by al-Qaeda."
Roque added that killing Nabhan could be counter-productive as he would be viewed as a "martyr".
"If the human intelligence was so good to have located Nabhan in two isolated trucks, they could have liaised with the Somali government and captured him.
"They could have had him extradited to Kenya to try him for crimes committed against its citizens.
"So, in one sense, it works in counter-terrorist strategy combating al-Qaeda. In another sense it doesn't as it creates national symbols."
Daniel Volman, a director of the African Security Research Project in Washington, echoed that sentiment.
"I have always argued that it is a mistake to treat terrorism as a military threat, because it does not pose a direct military danger to the United States.
"[Terrorism] is a terrible crime, and should therefore be treated as a law enforcement issue, in which case the objective should be to bring people to trial, to show that there is a case against them, to convict them and then legally imprison them - not to carry out illegal murders abroad like this," he told Al Jazeera.
Volman added that the raid, if confirmed to be carried out by the US, "shows that the Obama administration has decided to continue to pattern established under the Bush administration - of carrying out what can only be labelled assassination or murder without regard to international law".
Sheikh Bare Mohamed Farah Khoje, an al-Shabab spokesman, said after the attack that al-Shabab would continue its fight against the Western world.
"Al-Shabab will continue targeting Western countries, especially America ... we are killing them and they are hunting us," he told the Reuters news agency.
"We wish we could eradicate them all. We will never forget our brothers who were targeted illegally by the United States."
Nabhan, born in Kenya, was high on the FBI's list of most wanted "terror" suspects.
He was wanted for questioning in connection with the car bombing of an Israeli-owned beach resort in Kenya which killed 15 people and the near simultaneous attempt to shoot down an Israeli plane leaving Mombasa airport in 2002.
Nabhan was also suspected of involvement in recruiting some of the 500 foreign fighters currently believed to be in Somalia.
Ernst Jan Hogendoorn, Horn of Africa project director at the International Crisis Group, said the "surgical" precision of Monday's raid showed the US has specific intelligence in Somalia.
"I think it will certainly make al-Shabab leaders much more cautious when they are operating because obviously the United States has very precise intelligence about their movements," he said.
The US military has launched several air attacks inside Somalia in the past against individuals blamed for the bombings of the US embassies in Kenya and Tanzania in 1998.
In May last year, US aircraft killed Aden Hashi Ayro, the then-leader of al-Shabab and allegedly a senior al-Qaeda member, in an attack on the central town of Dusamareb.