ATHENS - Two foreign gunmen carrying explosives hijacked a Greek bus with 26 passengers on Wednesday and demanded to go to Russia, a freed hostage said.

Albanian and Russian diplomats were called by police to the scene to try to pin down the nationality of the gunmen, who took over the public transport vehicle in an Athens suburb early in the morning.

Some Greek media quoted the Albanian ambassador as saying the hijackers "may be Albanian", but there was no confirmation.

Hundreds of thousands of Albanians live in Greece, many of them having arrived to help with construction work for last August's Athens Olympics.

About five hours after the 6am hijack, five hostages -- three women and two men all appearing in poor health -- were freed by the gunmen.

Stella Matara, a hostage still on the bus, told state-run television in a mobile phone call that the hijackers planned to release all women prisoners once a driver joined the vehicle.

The original driver, a ticket collector and a woman passenger managed to escape from the bus in the first seconds of the hijack when shots were fired during the takeover.

"They want a police bus to leave from in front of our bus, as well as a driver to take them to the airport," Matara said.

"As soon as the driver comes, they will release all women. At the airport, they want a plane to take them to Russia, and then they will release the rest of the hostages.

"That's all. No other demands. No money, no nothing. They have not harmed us. They've not harmed us at all for now. They have guns they have dynamite. I can see them in front of me.

"I don't know what will happen later because both they (hijackers) and us are now very, very nervous."

The bus was seized along a highway from the Athens suburb of Marathon, used during the Olympic race of the same name this summer, police said.

The seizure of the bus was the first such incident since a spate of bus hijackings in Greece in 1999-2000.

Albanian Ambassador Bashkim Zeneli and the Albanian police attache, Agron Agalliu, as well as several Russian diplomats, were at the scene.

The first of the five hostages released, a grey-haired man who looked to be in his 50s, stepped out of the bus through the driver's side with his hands raised and walked about 30m before stepping over a low fence into the arms of waiting police.

"My Dad has a heart condition," Vassilis Bratsiakos said. "I am just happy he is well and far away from the bus."

Nikos Koutsogiorgos, head of the company that owns the commuter bus, told reporters he had spoken by mobile phone to one of the bus passengers who had relayed the gunmen's demands.

"A group of two people, both foreigners, took over the bus this morning," a police official told Reuters. "There were 26 passengers on the bus then.

"Shots were heard, police cars rushed to the scene and more shots were then heard."

Hundreds of police officers, snipers in camouflage attire and special forces took up positions around the vehicle. One police car, its lights flashing, had parked behind the bus.

"I don't care what they are or who they are. I want them to release my wife," said an elderly man who was among dozens of relatives of hostages who rushed to the scene.

"She told me she is fine and things are quiet on the bus but she sounded terrified."

There have been no reports of injuries, but live television showed one gunman approaching the front of the bus and firing off two warning shots.

The curtains in the bus windows were closed, blocking views inside, and a police helicopter hovered above. Television pictures had earlier shown one man carrying a rifle and standing inside the bus near the front seats.

Police blocked traffic along the busy eastern Athens highway leading to Marathon and kept news crews hundreds of metres away. Dozens of ambulances could be seen parked nearby.

This is the first such incident since the November 2000 hijack of a bus carrying 35 Japanese tourists who were taken hostage by a man who surrendered to a TV talk show host after a nine hour standoff. Previous hijackings, some involving Albanians, occurred in 1999.