Pro-Russia militants tried to take over the airport early on Monday morning. Photo: BBC

At least 30 pro-Russia separatists were killed, the insurgents say, after an attempt to take over the airport early on Monday.

New President Petro Poroshenko had vowed to tackle "terrorists" in the east "within hours, not months".

Meanwhile, the OSCE says it has lost contact with a monitoring team.

The Organization for Security and Co-operation in Europe said four of its monitors were on a routine mission east of Donetsk. Contact was lost at 18:00 (16:00 GMT) on Monday and has not been re-established.

A spokesman told the BBC the monitors were Turkish, Swiss, Estonian and Danish, and all were male.

Seven international military observers linked to the OSCE were taken captive in eastern Ukraine in April and held for a week before being released.

This family sought shelter inside a building near the airport as shots rang out

Ukraine's Interior Minister Arsen Avakov said in a statement on Tuesday: "The airport is under our full control. The enemy suffered heavy losses. We have none."

He said operations were still continuing. A BBC team there heard sporadic gunfire.

At the scene: Mark Lowen in Donetsk

The Kiev government was absolutely determined that the airport would not fall to the insurgents.

Back in February when pro-Russia separatists launched their incursion in Crimea - which led to Moscow's annexation of the peninsula a month later - the airport was the first key installation they took control of, so Kiev was committed to clamping down on that.

Ukraine's new president gave a speech on Monday saying he would not negotiate with "terrorists" as he put it - and clearly you see the impact of that around Donetsk airport.

A representative of the self-proclaimed Donetsk People's Republic told the BBC that reports of at least 30 dead in Monday's fighting were accurate.

Other reports of the death toll vary.

Donetsk mayor Alexander Lukyanchenko said 40 people had been killed in the past day, while another separatist spokesman said 50 fighters had died.

Photographs taken by the Associated Press from inside the morgue in Donetsk showed more than a dozen bodies in military fatigues piled in a small room.

President's vow

Monday's clashes started after separatist militants stormed the Sergei Prokofiev Donetsk airport in the early hours.

The Ukrainian military responded quickly with air strikes and an assault by heavily armed troops.

The separatists may have been trying to prevent a visit by the new president

The attempt to seize the airport may have been intended to prevent Mr Poroshenko from travelling there after he said his first trip would be to visit the restive east.

Mr Poroshenko, a 48-year-old billionaire and former foreign minister, was on Monday formally declared the winner of Sunday's presidential election with 54% of the vote.

He vowed east Ukraine would not be "turned into Somalia", adding: "The anti-terrorist operation cannot and should not last two or three months. It should and will last hours."

However, Mr Poroshenko has also said he wants to talk to Russia to end the crisis.

Before Sunday's election, Moscow had said it would accept the results and engage in dialogue with the winner.

But Russian Foreign Minister Sergei Lavrov said on Tuesday: "The question of a visit to Russia by Poroshenko is not being considered and is not being discussed through diplomatic or any other channels."

He again called on Mr Poroshenko to stop military operations in eastern Ukraine immediately and implement a roadmap for peace negotiated in Geneva on 17 April.

Speaking at a news conference in Moscow, he said a "real war" was under way in the eastern Donetsk and Luhansk regions.

The regions declared independence after Russia annexed the Crimean peninsula in the wake of the removal of Ukraine's pro-Moscow President Viktor Yanukovych.