A new US-backed military offensive against Islamist militants in Somalia could undermine the massive international effort to help millions of people threatened by the worst drought there in more than 40 years, aid officials in the unstable east African state fear.

More than £50m has been raised by individual donors in the UK and the British government has contributed another £110m to help avert hundreds of thousands of deaths in Somalia. More than six million people there are in need of immediate assistance, with half of them facing famine.

British officials in Mogadishu, Somalia’s capital, said that the effort was “unprecedented”. The UN target of raising $835m (£654m) has largely been met, raising hopes that a repeat of the tragedy of the 2011 famine, which killed 250,000 people in the country, will be averted. However, aid workers in Somalia warn that any significant offensive, especially if accompanied by the use of air power, could have a devastating effect on relief operations.

“Increased belligerence from some international and national actors is not going to help us … if things deteriorate as a result of military effort, that will be man-made,” said Peter de Clercq, the United Nations’ deputy special representative of the secretary-general in Somalia. “We have argued very strongly that this is not the time for military action.”

Other senior aid officials spoke of a “nightmare scenario” of widespread fighting in the middle of a humanitarian crisis. “It would be a catastrophe … and could totally undermine everything that is being done to save lives,” said one official, who requested anonymity because of the sensitivity of his organisation’s operations in the country.

Facebook Twitter Pinterest A mother and her child seek shelter at the Doolow refugee camp after fleeing drought in territory occupied by al-Shabaab. Photograph: Muse Mohammed//IOM

But Mohamed Abdullahi Mohamed, the newly elected president of Somalia, and President Donald Trump have both signalled an imminent offensive against al-Shabaab, an Islamist extremist group allied to al-Qaida that controls much of the region worst hit by the drought.

Mohamed, a dual US-Somali national, pledged earlier this month to deliver on his campaign promise to rid his country of the group. At a press conference last month, he offered an amnesty to al-Shabaab militants who surrendered within 60 days, but warned the rest would “face the consequences … a new war”. Trump recently designated the country a “zone of active hostilities”, allowing commanders greater authority when launching airstrikes, broadening the range of possible targets and relaxing restrictions on the use of air power designed to prevent civilian casualties.

Ministers said that Mohamed’s decision to launch renewed action against al-Shabaab had been taken after talks with Washington.

“There has been close consultation with the two countries and our president has called a state of war,” said Abdirahman Yarisow, the Somali information minister. “We are very serious about trying to carry out military operations to wipe out al-Shabaab from the country. It is do-able, but needs lots of resources, commitment from our friends and allies, and discipline.”

Trump has also authorised the deployment of regular US forces to Somalia for the first time since 1994. The US in effect pulled out of Somalia after 1993, when two helicopters were shot down in Mogadishu and the bodies of American soldiers were dragged through the streets.

The US president has said that defeating radical Islamist terror groups is the “highest priority” of his administration, and that the US “will pursue aggressive joint and coalition military operations when necessary”.

Al-Shabaab has not been implicated in any plots to strike the US or Europe, but has carried out several high-profile terror attacks in the region and has attracted recruits from the US as well as Europe. Even before he took power, Trump’s advisers asked State Department officials dealing with east Africa why, after years of effort, the war against al-Shabaab had not been “won”.

Mohamed has promised voters he would wipe out the movement within two years. Yarisow said: “All this … will give us an opportunity with our allies and friends to really target al-Shabaab from the air, and for our federal forces and [the regional military forces in Somalia] to take advantage.”

Millions of those needing emergency assistance to avoid starvation live in areas controlled by al-Shabaab. These zones are likely to bear the brunt of any new fighting. The timing of military action is likely to concern some US allies, too. Senior officials in Britain’s Department for International Development and the Foreign Office favour low-level negotiations with al-Shabaab to allow access to drought-hit areas, minutes of recent meetings seen by the Observer show. This would almost certainly be jeopardised by any serious fighting.

Violence is already a major obstacle to humanitarian relief in Somalia, with more than 100 killed in terrorist attacks in Mogadishu this year, and daily ambushes of government or international forces by al-Shabaab. Last week aid workers from the World Food Programme and the United Arab Emirates were targeted in two separate attacks on the outskirts of the capital.

More than 50 al-Shabaab fighters were reported killed last month by Kenyan troops deployed in Somalia as part of Amisom, a regional stabilisation force of about 22,000 men.

Al-Shabaab, which does not allow most western organisations to enter territory it controls, dismissed the amnesty as a “fraud designed to please the west” and has promised to meet any offensive “with redoubled force”. The group currently allows people to leave its area of control to seek medical assistance, food and shelter – but there are fears that this could change.

Facebook Twitter Pinterest Goats being watered at Bali Hiile, Doolow. Photograph: Muse Mohammed/IOM

“At the moment al-Shabaab is allowing freedom of movement, but the declaration of war was very badly timed. It may change the dynamic. The mortality rate could be massive,” said another aid official, who runs operations in Somalia for a major international NGO. “Our original hope and expectation was of a ceasefire. That would have given us a vital two to three months to reach two to three million people living in areas that are inaccessible.”

An offensive would be likely to rely heavily on US air power and possibly special forces. Military and counterterrorism advisers have been present in Somalia for several years, working with local forces. A small unit of special forces has participated in raids on al-Shabaab.

Such troops would play a vital role in any renewed fighting. The Amisom stabilisation force has shown little appetite for major operations against al-Shabaab in recent years and has suffered significant casualties in attacks by the group.

The US has steadily intensified its war in Somalia. According to data compiled by the thinktank New America, there have been 41 strikes by US forces in Somalia since 2003, with al-Shabaab a target since 2008. The number of drone strikes has risen sharply since 2015. A single drone strike last year killed an estimated 150 al-Shabaab fighters.