The United States government on Saturday warned its citizens of an impending terrorism attack in Kenya's coastal city of Mombasa.

In a statement issued by the US embassy in Nairobi, Washington ordered Americans to vacate Mombasa as it also suspended all US government travel to the port city until July 1, over terrorism fears.

"US private citizens are not subject to the same restrictions but should consider this information in their travel planning. All US government travel to Mombasa is suspended until July," the statement said released on Saturday.

"This is to alert all US citizens in Kenya, or planning to travel to Kenya in the near future," the American embassy said in its third alert this year of imminent threats of terror attacks.

The move came three days after the police arrested two Iranians with dangerous explosive bomb materials that could be used to carry out terror activities in the east African nation.

Mombasa regional police commander, Aggrey Adoli said the two terror suspects, all Iranians, were arrested by anti-terrorism police immediately after checking into a hotel in Nairobi on Tuesday night and later flown to the coastal city of Mombasa for interrogation.

"We can confirm that indeed we recovered explosive materials weighing 15 kilograms in powder form and bomb experts have collected the samples for further forensic examination," Adoli said on Thursday.

The suspects are being questioned by a special team of detectives from Nairobi and Mombasa as police pursue their accomplices and more explosive materials and devices.

However, Kenyan Police Commissioner Mathew Iteere said the security agencies have been working round the clock to ensure Kenyans enjoy maximum security and allayed fears that the country was facing an imminent terror attacks.

"There is no cause for alarm. We have been working round the clock to ensure there is enough security in the country. We have been also preventing several terror attempts in the country linked to the terror groups," Iteere said.

The police chief said they have been working with several foreign security forces to ensure the east African nation is safe from terror threats.

Kenya has witnessed several hit-and-run grenade attacks in recent months in the capital, Nairobi, and in northern Kenya and Mombasa port city. The Kenyan authorities often blame such attacks on Al-Qaida-affiliated Al-Shabaab rebels from neighbouring Somalia.

The East African nation launched cross border incursion into Somalia last October in pursuit of the militants blamed for a series of grenade and landmine attacks in Nairobi, Mombasa and northern region.

The authorities have called on Kenyans to be vigilant over terror attacks because of higher numbers of foreign fighters who sneaked into the country.

The security agencies have also been on higher alert after intelligent reports hinted that Al-Shabaab terrorists are escaping into the East African nation as the operation to rout the insurgents from Somalia gains momentum.

Police sources told Xinhua that said more border patrols have been intensified along the Kenya-Somalia border in Kiunga and Dadaad refugee camps to combat the Al-Qaida-linked insurgents to gain entrance into the country.

This recent arrest and recovery of bomb materials also came after police raided a warehouse in Mombasa port city after intelligent reports of container loaded with explosive material was sneaked into the country.

A team of anti-terrorism police officers are still scanning several containers at the warehouse in Mombasa port city.

The country's anti-terrorism team who were led by regional commander Elijaj Rop raided the containers that had been tracked for several weeks before entering Mombasa port.

Police believe the suspects are part of a regional terror syndicate and maybe in the East African nation plotting another deadly attack on the Kenyan soil.

Top US military officials have in the past expressed concerns that Al-Shaabab intends to connect with other al-Qaida-linked militant groups in Africa.

Head of Anti terror unit in Kenya Njiru Mwaniki said the security forces are pursuing some of the foreign fighters who operate terror cells in country.

A top German terrorism suspect, who was being hunted by Kenyan police following a blast in the capital Nairobi last month, was arrested in neighboring Tanzania last week and later deported to Germany.

The man identified as Emrah Erdogan, a German national of Turkish origin who is also known as Salahuddin al-Kurdi had escaped police dragnet in Nairobi capital and Mombasa port city before he was arrested in Tanzania.

Kenya sent troops into Somalia to battle Shabaab rebels in October 2011 after several attacks, including the kidnapping of a French woman and a British tourist ?C and the killing of her husband ?C damaged its key tourism industry.

The Somali militants, who deny involvement in the coastal attacks, in turn vowed to attack Kenya. Britain has also travel advisory to its citizens to be on a high alert over terrorist attacks in Nairobi.

Several people have been killed in northeast regions bordering Somalia since troops launched a cross-border campaign against the Al-Shabaab, including grenades thrown into bars and a church, and several homemade explosive devices and landmines set off.

Police now believe terror suspects would not operate freely unless permitted by their local host communities.