No reason given for president's cancellation of visit to Chibok, which would have been his first since abductions on 14 April

The Nigerian president has cancelled a visit to the northern town from where more than 200 schoolgirls were kidnapped last month, as the US said the rescue mission was proving tough.

Goodluck Jonathan had been preparing to make his first visit to Chibok since the mass abductions were confirmed. He has been criticised for a slow response to the crisis and for failing to visit the girls' home town.

No reason was given for the cancellation, but security is likely to have been a major concern. The area is a stronghold of Boko Haram, the Islamic militant group that snatched the girls from a school on 14 April. Insurgents killed at least four soldiers in an ambush on Monday.

Relatives in Chibok were angry at the cancellation, saying they had no confidence that the government was doing everything possible to find the missing girls.

"You begin to question what could be more important to the president than the lives of these students," said Allen Manasseh, whose 18-year-old sister Maryamu Wavi was abducted from the Chibok government girls secondary school.

"The parents were hoping he would come with some information for them about where the girls may be and what efforts are being done to recover them, but instead to be told he is not coming was not easy for them. It's not an easy thing to have a missing child."

Manasseh said his sister was not one of the girls pictured in the video released by Boko Haram. "It makes me very worried for her, because I do not know whether she is alive or where she might be."

He said the families were upset that it had taken more than a month for Jonathan to schedule a visit to Chibok, but that promising to come and then postponing without an explanation gave them even less faith in the authorities.

In Washington, the state department spokeswoman, Marie Harf, said Nigerian and international teams attempting to find the girls in the remote Sambisa forest faced "a tough challenge".

The US has deployed manned and unmanned surveillance aircraft over the area. According to the New York Times, 30 specialists from the state department, FBI and Pentagon, with medical, intelligence, counter-terrorism and communications expertise, have been sent to Nigeria.

The White House spokesman, Jay Carney, said there would be no active deployment of US forces in Nigeria.

The US defence secretary, Chuck Hagel, told CBS News it was an open question whether Nigerian forces were capable of rescuing the girls.

The US general David Rodriguez met Nigerian officials in Abuja this week, as did the British Foreign Office minister Mark Simmonds.

Jonathan was due to fly to Paris on Friday for a summit hosted by François Hollande, the French president, to discuss the security threat posed by Boko Haram. Nigeria's neighbours, Benin, Cameroon, Niger and Chad, were expected to attend, along with US, UK and EU representatives.