Internally displaced families board trucks as they travel back to their home regions from Ala-yasir camp closed by the al Shabaab militias, in Lower Shabelle, 50 km (31 miles) south of Somalia's capital Mogadishu, October 15, 2011. [Photo/Agencies]

NAIROBI - Kenyan combined security forces have crossed the border into Somalia to pursue militants of Al Qaida-linked group al Shabaab.

Police sources confirmed on Sunday evening the joint operation was launched with agreement with Somali government forces to hunt Islamic groups who kidnapped two Spanish, French and one British women in the past few days.

"Yes, the joint operation is underway in search for al Shabaab militia who have been blamed for recent abductions of foreign in northern Kenya," a senior security official told Xinhua on condition of anonymity.

Eye witnesses said the country's military planes and helicopters overflew the border with Somalia as troops and their paramilitary counterparts around Liboi crossed the frontier to engage the militants.

The witnesses said the country's military crossed into the Horn of African in Mandera with orders to fight their way into the lawless country and create a buffer zone to ensure that insurgents do not launch attacks against Kenya.

The witnesses said they saw military trucks at border points and choppers in the air which are said to have bombed al Shabaab positions on Sunday.

They said the troops crossed more than 100 km into Somalia in hot pursuit of the kidnappers of two Spanish aid workers from the Dadaab refugee camp, the world's largest refugee camp.

"There is fierce fighting going on inside Somalia. Residents are aware about the operation which has been launched by Kenyan troops inside Somalia," Wycliffe Kimani, a teacher in Mandera told Xinhua on Sunday evening.

The latest crackdown on Somalia militia group came after the government invoked Article 51 of the UN chapter that gives a country the go ahead to defend itself against any aggression in its territory.

Speaking earlier on Sunday, Internal Security Minister George Saitoti said that the article further gives a state the right to enter into a foreign country to prevent any such occurrence and that the action aims to push al Shabaab military forces as far away from the border as possible.

Saitoti confirmed that the East African nation which has been hosting thousands of Somali refugees has launched a spirited offensive on the al Shabaab and will not relent until the country's territorial boundary is secure.

"By invoking article 51 of the UN Charter, Kenya has the backing of the UN Security Council to pursue the al Shabaab into Somalia," the security minister said during a tour of Isiolo and Tigania East on Sunday.

"We are on the trail of the al Shabaab and we are going to extinguish the al Shabaab militias that have visited suffering on innocent Kenyans and foreigners," he said.

The al Shabaab threat is very real and grave and it is undermining our national security as well as our economy.  

He said the al Shabaab militia has in the recent past abducted four foreigners from inside Kenya undermining the economy of the country, adding that the militias abducted two military personnel who since July have not been unaccounted for.

The two Spanish women, Montserrat Serra, age 40, from Girona (Palafrugell) and Blanca Thiebaut, age 30, from Madrid, both working as logisticians for MSF in the Dadaab refugee camp were abducted on Thursday in Daddab refugee camp.

Sources near the common frontier said early on Sunday that Kenyan forces are assembling at the border points where they are getting briefs and other relevant instructions as they prepare to enter the lawless country which has been not without a central government for more than two decades.

The sources said the combined security forces are under firm instructions for the next assignment which will mainly include pushing the al Shabaab rebels far away inside Somalia from the common border.

But speaking at the local television, Permanent Secretary in the Ministry of Internal Security Francis Kimemia said a large contingent of Kenyan troops have been deployed to hunt for the al Shabaab and push them inside Somalia.

"Now we can't wait for the Transitional Federal Government of Somalia or Amisom. Article 51 of the UN charter allows us to pursue them. It allows you to hit anybody who hits you or is planning to hit you," Kimemie told Citizen Television on Sunday evening.    

"And also allows you to pursue those who have hit and ran away, "  Kimemia said, adding that the country has been under sustained provocation from al Shabaab for months, with the al-Qaida-linked group raiding across the border.

The PS said it was the militant group which has declared war against  Kenya, "we have subsequently done the same and there no is turning back."

Defense Ministry said late on Sunday that Kenya was using various options in pursuing the al Shabaab militants.

"We have a long border with Somalia which stretches 1,400 kilometers from Mandera to Kiunga and we are exploring aerial, naval and ground offensive options," the ministry said in statement.

The statement which was issued after a long security meeting with relief agencies said other international forces in a "concerted effort and rescue operation" joined Kenya.

Aid agencies working in northern Kenya and in Somalia said the latest attack is jeopardizing the assistance to thousands of people in urgent need of humanitarian aid and a quick and satisfactory solution is necessary.

United Nations staff and international aid workers often travel with armed Kenyan police escorts from their bases to the various camps. They are not allowed, under their regulations, to travel on their own.

However, MSF usually operates without a security escort. The medical humanitarian organization delivers emergency aid to people affected by war, epidemics and disasters.

The kidnapping comes weeks after Somali gunmen abducted a British woman and a French woman from beach resorts in separate incidents along the northern Kenyan coast. Both women are being held hostage in Somalia.