With Russian armoured personnel carriers on the move in the Crimean peninsula, world leaders have sought assurances from the Kremlin that Moscow is not acting to escalate the violence in Ukraine.

A convoy of nine APCs painted with the Russian flag were seen on the road between the port city of Sevastopol and the regional capital of Sinferopol. Reporters spotted them parked on the side of a road near the town of Bakhchisarai, apparently stalled after one vehicle developed a mechanical fault.

The Russian foreign ministry said movements of vehicles belonging to the Russian Black Sea Fleet were prompted by the need to ensure the security of its base in Sevastopol. Russia is supposed to notify Ukraine of any troop movements outside the naval base. The Ukrainian defence ministry said it had no information about the vehicles' movements.

John Kerry, the US secretary of state, confirmed he had spoken to the Russian foreign minister, Sergey Lavrov, on Friday, hours after unidentified soldiers seized two airports in the Ukrainian peninsula overnight.

Kerry said the US was watching to see if Russian activity "might be crossing a line in any way" and had urged the Kremlin against action that might be misinterpreted as a violation of Ukrainian sovereignty.

Armed men patrol at the airport in Simferopol, Crimea. Photograph: Viktor Drachev/AFP/Getty Images

Military troops in unmarked uniforms resembling Russian uniforms took over two airports in Crimea, Simferopol airport and a military facility at Sevastopol, overnight, and there were reports on Friday evening that Simferopol airport was not allowing flights from Kiev.

After the airport seizures, Andriy Paruby, the newly appointed top Ukrainian security official, accused Russia of waging "a military invasion and occupation". "These are separatist groups … commanded by the Kremlin," Paruby said of the armed military men patrolling streets in the Crimean cities of Simferopol and Sevastapol.

Moscow has denied launching a military offensive in the region.

Journalists and the Ukrainian border guard have reported a fleet of more than 10 Russian military helicopters entering Ukrainian air space over Crimea, flying from Russia.

British and German leaders have also called the Russian president, Vladimir Putin.

A Downing Street spokesperson told Reuters: "The prime minister emphasised that all countries should respect the territorial integrity and sovereignty of Ukraine. President Putin agreed, stressing that Russian military exercises in the area had been planned before the current situation in Ukraine. They agreed that the free and fair elections that the interim government has pledged to hold are the best way to secure a positive future for the Ukraine."

As newly appointed members of the Ukrainian parliament appealed to the UN security council to call a session to address the crisis, the Russian parliament began considering a law that would allow Moscow to add new territories in a simplified manner. There are fears that the Kremlin, unhappy with the events that led to president Viktor Yanukovych fleeing Kiev last weekend, is trying to sow unrest in the largely pro-Russian region.

Adding to the sense of alarm, the interim Ukrainian president, Oleksandr Turchynov, dismissed the head of the armed forces, Admiral Yuriy Ilin, on Friday..

Paruby, the secretary of the National Security and Defence Council, said a state of emergency may be declared: "It is one of the options in the development of events."

Crimea graphic

This contingency would not necessarily involve the deployment of the army, he said. "We are looking for other more effective ways of localising the situation on the Crimean peninsula."

According to Ukrainian legislation, a state of emergency can be introduced by parliament for up to two months.

In Simferopol, groups of armed men arrived overnight at the main airport serving the region. They wore military fatigues with no insignia and refused to talk, though one told news agencies they were part of a self-defence unit who wanted to ensure that no "fascists" arrived in the region from Kiev.

At Sevastopol airport, a military airport that handles few commercial flights, a man who said he was a captain in the tactical aviation brigade but declined to give his name, told the Guardian there were about 300 people of unknown identity inside the airport. "We don't consider it any invasion of our territory," he said without elaborating.

He said the men looked like military, were wearing two different types of uniform and were armed with sniper rifles and AK-47s. "We don't know who they are, nor where they've come from."

He added that there were two large trucks inside. "They [the vehicles] looked like they could contain 50 people at a push, so how they got 300 people inside I don't know," he said

A Major Fidorenko, from the Ukrainian military based at the airport, said the Ukrainians had been in touch with the unknown gunmen, who said they were there "to prevent unwanted landings of helicopters and planes".

In Kiev, the new interior minister, Arsen Avakov, wrote on Facebook: "I can only describe this as a military invasion and occupation."

On Thursday, masked gunmen with rocket-propelled grenades and sniper rifles seized the parliament building and government offices in Simferopol. The Russian flag was raised above the parliament.

Yanukovych gave a press conference in the southern Russian city of Rostov-on-Don on Friday afternoon, saying Crimea should remain part of Ukraine and claiming he was still president. Russian government sources say they have offered "protection" to Yanukovych in Russia, though it remains unclear how he arrived in the country.

In further worrying signs for Kiev, Russia's parliament began considering two new laws on Friday. One of them offers eased citizenship requirements for Russian-speaking Ukrainians, removing the requirement that they should have lived in Russia for an extended period, while the other makes it easier for Russia to add new territories to its existing boundaries.

The latter law, which appears to be aimed pointedly at the Crimea situation, says territories can be added by a local referendum "in the case that a foreign country does not have effective sovereign state authority".

Unidentified gunmen wearing camouflage uniforms guard the entrance to the military airport at Sevastopol in Crimea. Photograph: Ivan Sekretarev/AP

Sergei Aksenov, the new prime minister of Crimea, has said he does not recognise Kiev authorities and still believes Yanukovych to be the legitimate Ukrainian president.

At a roadblock on the way from Simferopol to Sevastopol, Vladimir Zhirinovsky, a nationalist Russian politician, said Russia was not planning to take Crimea. However, he made it clear that the Russians still saw Yanukovych as Ukraine's legitimate president.

"Russia will help the economic situation in the south-east. We can provide fuel and gas at a cheap price," he said. "The [military] manoeuvres in Russia were normal, they were nothing special. Crimea should decide its future for itself. We Russians don't want to create the impression we will take Crimea. Yanukovych is the lawful president by the constitution and the only one who can sign a decree."