Shots fired at observers, Russians pictured laying landmines, war of words intensifies.
A Ukrainian border patrol plane came under fire near the regional boundary with Crimea as tensions increased further in the contested peninsula yesterday.
The Diamond light aircraft was flying three crew on an observation mission when shots were fired. No one was injured.
But the confrontation, coming just hours after Russian forces reportedly laid landmines along the border and fired warning shots at a European observer mission from the Organisation for Security and Co-operation in Europe (OSCE), provided worrying evidence of the rapidly rising temperature in Crimea.
The OSCE convoy was trying for the second day to enter the peninsula from the north. Three bursts from an automatic weapon were fired into the air as checkpoint commanders insisted that the OSCE had no authorisation to enter. Witnesses said the shots were fired by men in balaclavas.
The OSCE mission, consisting of more than 50 officers from European armies, was sent by the Vienna-based organisation to address Russia's claims that Russian speakers in Crimea were being threatened by the new Western-backed government in Kiev.
Meanwhile, an investigative Russian newspaper published photographs of lines of landmines being laid near the second entry point to Crimea close to the villages of Chongar and Novooleksivka.
Evgenii Feldman, the Moscow-based photographer who took the picture, said: "The villagers don't want to be on the front line. In Chongar, the school has been closed since Wednesday. On the internet, locals are talking about the impossibility of studying under the fire of armoured personnel carriers."
Ukraine's Foreign Minister again called for direct negotiations with Russia to resolve differences over the revolution that provoked the collapse of the Moscow-linked government of President Viktor Yanukovych.
"Our priority is a peaceful settlement of the Crimea conflict," said Andriy Deshchytsa. "We will not give up Crimea and will do all possible to protect Ukrainian borders and territorial integrity."
But Sergei Lavrov, the Russia Foreign Minister, condemned attempts to portray Russia as responsible for confrontation in Ukraine and said the Kremlin would not negotiate until it was treated differently.
"This crisis was not started by us. We are ready to continue dialogue on the understanding that this dialogue should be honest and partner-like, without attempts to portray us as one of the sides in the conflict."
Crimea's officials pledged to press ahead with its proposed referendum on joining the Russian Federation this weekend. Sergei Aksyonov, the premier of the region, said: "No one is able to cancel it. It has been called at short notice to avoid provocations, as the situation in Ukraine is quite tense".
Russia also warned that it had the grounds to stop American inspections of its nuclear arsenal as authorised by arms controls treaties.
"We are ready to take this step in response to the announcement by the Pentagon about stopping co-operation between the defence institutions of Russia and the United States," a statement said.
The developments came as Michael Flynn, head of the US Defence Intelligence Agency, said it had detected a Russian troop build-up for "seven to 10 days" around Crimea before the Kremlin seized control of the area.
As the Russian forces in Crimea solidified the divisions between the territory and Ukraine, pro-Russian protesters pressed their claim for a vote on their future across the eastern half of the country.
In Ukraine's eastern cities, which are dominated by Russian speakers, hundreds of people gathered in squares to denounce the new Kiev government as a front for extreme right-wing movements and call for a vote on their future status.
In Donetsk, hundreds of Russians rallied to support Pavel Gubarev, the local protest leader who was arrested on Saturday after proclaiming himself the people's governor. Organisers called Gubarev a victim of political repression and demanded the release of the 30-year-old businessman.
"We are calling for the freedom of Pavel," said organiser Robert Donia. "We want a vote on the status of the region. The people are for a referendum."
A couple on a platform offering a book to sign up for a referendum were doing a brisk trade.
Lilila Khardasheva, a doctor, said eastern Ukraine needed a vote on autonomy because people were fearful of militant elements in the pro-Western coalition in Kiev. She said Donetsk was putting its faith in Russian President Vladimir Putin.