Russian soldiers dig trenches and set up mortars on a hillside overlooking the Ukrainian military base of Perevalne in Crimea.

Inside, soldiers loyal to Kiev are facing a painful dilemma -- desert and submit to the new authorities or leave the peninsula.

Crimea's separatist parliament this week ordered the "dissolution" of these military units in Crimea as part of a declared nationalisation of Ukrainian state property.

The bases have been under siege since February from pro-Russian civilians and thousands of Russian soldiers without national insignia.

Crimea's parliament speaker Volodymyr Konstantynov has ordered the Ukrainian military to "serve in Crimea and swear allegiance to the republic or continue to serve outside the borders of Crimea in the Ukrainian army".

Thousands of Ukrainian military are based on the peninsula -- Russian President Vladimir Putin on Tuesday estimated their number at 22,000 -- who have been forced to watch listlessly as Russian forces cement their siege.

Ukrainian authorities on Monday reiterated there was no question of their troops leaving the peninsula but have excluded the possibility of attacking the Russian forces.

Reports of an agreement between Russian and Ukrainian commanders on the partial lifting of the siege until Friday to allow supplies did not appear to be reflected on the ground in Perevalne -- a base 25 kilometres (16 miles) south of Crimea's main city, Simferopol.

Outside the gates, around 15 Russian soldiers in camouflage could be seen near an armoured personnel carrier, lines of barbed wire and cement blocks to prevent access.

The siege has been getting more organised by the day -- with heated tents, toilets and transmission equipment now set up near the base.

- Defend without shooting -

Pro-Russian civilians in front of the gates were more aggressive than ever, insulting and threatening foreign journalists and trying to take a camera from an AFP reporter.

Three Ukrainian soldiers observed the scene.

At the immense base of Sevastopol, home to Russia's Black Sea Fleet, just two ships are flying the blue and yellow colours of Ukraine -- compared with around 20 Russian flags.

The Ukrainian ships have distanced themselves from the dockside to make an assault more difficult, and three marines with bulletproof vests and helmets could be seen standing guard.

Russian vessels and giant cisterns linked by chains block access to the port.

"We are prisoners of the political situation and of the port," said Pavlo, an officer.

"Until the politicians decide the situation between themselves, we will be hostages here," he said, explaining that he had been ordered to defend the ship without firing a shot.

"We have an agreement with the Russians that everything stays calm until March 21, and after that as well I hope," he said.

"After that we don't know, only Moscow and Kiev know. We're here, we're not getting off the ship. We're military men, we're waiting for our orders," he said.

In Sevastopol and Simferopol, special offices have been set up inside the recruitment centres of the Ukrainian army -- now under Crimean control -- for the soldiers and sailors of Kiev who want to defect.

A day after the referendum, Crimea's prime minister Sergiy Aksyonov said that 500 Ukrainian military in Sevastopol had already joined Crimea's forces -- soon to be the Russian army.