Nairobi - A massive security operation is in place to protect President Barack Obama during his visit to Kenya, which has been frequently targeted by al-Shabaab militants based in neighbouring Somalia.Ahead of Obama's arrival on Friday evening, large numbers of security forces patrolled in the Kenyan capital, Nairobi. Several US military aircraft were spotted flying around the city. There have also been significant military attacks on Islamic militant targets in Somalia in recent weeks.Major Nairobi roads will be temporarily closed and authorities said the international airport will be closed at times coinciding with Obama's landing and departure Sunday for Ethiopia. Safaricom, a mobile network operator, warned of disruptions while Obama is in Nairobi to meet entrepreneurs and hold talks with Kenyan President Uhuru Kenyatta."We recognise the fact that as the most protected individual in the world, there will be some security measures undertaken by his team which could include the temporary disruption of mobile signals close to where the President is at any given time," Safaricom said.Al-Shabaab, which is linked to al-Qaeda, has conducted major attacks in Kenya, including the 2013 attack on Nairobi's Westgate mall and an April attack in Garissa town that killed nearly 150 people.This month, Somali officials said African Union and local troops seized Bardhere, one of the last major towns held by extremists in Somalia's southwest. A militant commander was also killed in a US drone strike, according to Somali and US officials.A Somali intelligence official told The Associated Press that the use of surveillance drones has increased in the past two weeks."They are watching militants' movements towards Kenya so closely," said the official, who requested anonymity because he is not authorized to speak to the media.