Russian officials say a water shortage in Crimea is threatening to become acute as Ukraine has reduced the supply via a key canal.
Ukraine does not recognise the new authorities in Crimea who are backed by Moscow. Russia made the peninsula part of its territory last month.
Crimea's harvest of grapes, rice, maize and soya will be ruined if it does not get more water soon, officials say.
Russia says the Crimea-Ukraine border is now officially a state border.
The Russian government plans to establish permanent checkpoints there, as well as new rules for entering or leaving Crimea, Ria Novosti news agency reports.
The North Crimea Canal delivers water to Crimea from the River Dnieper, in Ukraine's southern Kherson region. The canal accounts for 80% of Crimea's water.
The current water shortage is threatening 120,000 hectares (296,000 acres) of Crimea's crops, which rely on irrigation, Russian Agriculture Minister Nikolai Fedorov said.
A ruined harvest across that area would mean losses of up to 5bn roubles (£83m; $140m), he told the Gazeta.ru news website.
Row with Kiev
A BBC reporter in Crimea recently said the water supply was one of the chief concerns of local people, ahead of the controversial March referendum on joining Russia.
The canal authorities in Ukraine say Crimea has accumulated a huge debt for water supplied last year. The dispute is aggravated by the breakdown in relations between Kiev and Moscow.
The water supply to Crimea has diminished from 50 cu m (1,765 cu ft) per second to about 16 cu m per second, Crimea's new pro-Russian authorities say.
To deal with the shortage, new wells could be dug or water could be brought in from Russia, but such options are expensive, officials warn.
Masked Russian-speaking gunmen blockaded Ukrainian military units in Crimea last month, leading to the peninsula's annexation by Russia - a move condemned internationally.
Russia's President Vladimir Putin later admitted that regular Russian forces had helped pro-Russian militants in Crimea.
Most of the local population is Russian-speaking and Sevastopol is the base of Russia's Black Sea Fleet.