A search is under way in Kenya for two Ugandan military helicopters which have gone missing with 10 crew members on board, Uganda's army spokesman says.The helicopters were among four which were due to make a stop in Kenya, but only one landed in the town of Garissa.The other helicopter made an emergency landing in the Mount Kenya area and its five crew have been rescued.The aircraft were part of a contingent being sent to reinforce the African Union force in Somalia.Ugandan military spokesman Lt Col Felix Kulayigye said the Kenyan air force and other authorities were looking for two missing Mi-24 attack helicopters with the help of Ugandan officials.One Mi-17 transport helicopter made it safely to Garissa with 13 people on board, he told journalists in the Ugandan capital, Kampala.All the aircraft and personnel on board were due to spend Sunday night in Garissa, before flying on to Somalia.Earlier, Kenyan defence department spokesman Bogita Ongeri told the BBC the land and air rescue operation was being hampered by bad weather.Ugandan troops are part of the African Union force fighting the Somali Islamist group al-Shabab, which pledges loyalty to al-Qaeda.Despite losing ground to the AU and other forces supporting Somalia's UN-backed government over the past year, al-Shabab still controls many southern and central parts of the country.The helicopters were being sent for an offensive against al-Shabab later this month in the biggest town it still holds - the port of Kismayo, the BBC's Catherine Byaruhanga in Kampala says.Al-Shabab has staged deadly attacks in Uganda, which is the largest contributor to the AU force. A double suicide attack in 2010 killed 76 people watching the football World Cup final on TV in a bar and restaurant in Kampala.