Israel has taken the leading role among foreign countries in aiding and advising Kenyan forces after al-Shabaab Islamist extremists attacked the Westgate shopping mall in Nairobi, according to security and intelligence sources.

A host of other countries have also offered help, including Britain, which has resident MI6 officers in Kenya and special forces training their counterparts in the country.

Metropolitan police anti-terrorist officers are in Kenya to try to monitor the activities of suspects including Samantha Lewthwaite, the widow of the 7/7 London suicide bomber Germaine Lindsay.

The CIA also has officers in Kenya. However, counter-terrorist officials pointed to Israel as the country that has built up extremely close security co-operation as well as commercial ties with Kenya over more than a decade.

Israeli officials kept a public silence about possible involvement in the mall standoff. However, a defence ministry official told the Guardian that a team from Israel's elite counter-terrorism unit had assisted the Kenyan authorities in handling the hostage crisis.

An Israeli border police unit known as Yamam is specially trained in civilian hostage rescue operations.

An Israeli army spokeswoman responded to questions on Israeli involvement with a formula previously used with reference to covert Israeli activities, such as recent air strikes in Syria. "We don't comment on foreign reports," she said.

Reuters news agency quoted an Israeli security source as saying Israeli advisers were helping Kenya with the "negotiating strategy" to help end the siege.

"There are Israeli advisers helping with the negotiating strategy, but no Israelis involved in any imminent storming operation," said the source, who asked not to be identified.

The Israeli foreign ministry said later four of its nationals were in the mall at the time of the attack, and that two fled while two others were rescued. One was lightly wounded, treated and released. The upmarket Westgate shopping complex is part Israeli-owned.

In London on Monday, the defence secretary, Philip Hammond, chaired a meeting of the government's Cobra emergencies committee, as David Cameron cut short a visit to Balmoral to chair a second meeting later in the day.

Downing Street said the prime minister – who has spoken to the Kenyan president, Uhuru Kenyatta – offered help in terms of "policing, intelligence collaboration and other related kinds" of assistance.

UK staff from Addis Ababa in Ethiopia travelled to assist local officials with the efforts in Nairobi and a rapid deployment team was sent from London, the Foreign Office said.

There was no confirmation of any Britons being among those who carried out or planned the Kenya attack, though UK-based individuals, at one time estimated to number 50, have gone to Somalia to join al-Shabaab, the terror group believed to be behind the atrocity.

Intelligence sources said it was clear the attack had been carefully planned over a long period with the help of a network of al-Shabaab sympathisers in Kenya.

The Labour leader, Ed Miliband, offered his support to the government and said: "Those who carried out this attack will be condemned across the globe. The cold-blooded killing of innocent women, children and men is as despicable as it is shocking."