Al-Shabab, in alliance with the Hizbul Islam group, has vowed to topple the government of Sharif Ahmed, the president, and has stepped up attacks in recent weeks.
Surge of violence
About 200 people have been killed in a surge in violence this month and tens of thousands of people have fled the fighting in the capital.
"This is now a threat not only to the government, but to the Somali nation and sovereignty," Omaar said.
"So for us it is a war, and it is a war that needs to be won."
The Somali government has said that foreigners have joined al-Shabab in an attempt to "turn the country into an Iraq or an Afghanistan".
Patrik Peter, a Swedish security service spokesman, said on Friday that at least 20 young men with passports issued in the Scandinavian country had travelled to join groups in Somalia.
Ban Ki-moon, the UN secretary-general, on Thursday condemned the violent campaign to overthrow Somalia's "legitimate government".
"The secretary-general believes there is a unique window of opportunity for peace in Somalia, but the situation is fragile and international assistance is needed now," Marie Okabe, the UN deputy spokeswoman, said.
The situation in Somalia is further complicated because pirates in and around the Gulf of Aden are preventing much needed goods and supplies from getting through, including rice and pharmaceuticals.