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US President Barack Obama has urged his Russian counterpart Vladimir Putin to stop the flow of weapons into Ukraine and halt support for separatists.
The White House said that in the phone call Mr Obama warned of further sanctions if Moscow failed to act.
The Kremlin says Mr Putin urged direct talks between Kiev and the rebels in the east. Russia denies arming them.
The separatists have said they will observe a truce initiated by Ukrainian President Petro Poroshenko.
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.
Pro-Russian forces have taken control of two naval bases in Crimea – including the HQ of Ukraine’s navy.
Streams of Ukrainian servicemen were seen leaving the bases. Kiev said its navy chief has been detained.
Crimean leaders signed a treaty with Moscow on Tuesday absorbing the peninsula into Russia.
Sunday’s referendum approving Crimea’s split from Ukraine came nearly a month after Ukraine’s pro-Russian President Viktor Yanukovych fell from power.
Crimean and Russian officials said the vote showed overwhelming public support for joining Russia, with 97% of voters in favour.
But the West and the Ukrainian government in Kiev say the hastily organised referendum – boycotted by many of Crimea’s Ukrainian and Tatar minorities – was illegal and will not be recognised.
A passenger plane plunged vertically when it crashed onto an airport runway in the Russian city of Kazan, killing all 50 people on board, Russia’s transport minister says.
Investigators have found both of the flight recorders, or “black boxes”.
The Boeing 737 was arriving from Moscow and trying to land, but exploded on impact at about 19:20 local time (15:20 GMT) on Sunday, officials said.
The Tatarstan Airlines jet had 44 passengers and six crew on board.
The crash happened as the 23-year-old plane was making a second attempt to land. Earlier that day it had flown to the Armenian capital Yerevan and back to Kazan, then to Moscow.
The BBC’s Daniel Sandford: “This is the first time that Greenpeace have found themselves at the criminal end of the piracy law”
Russian prosecutors have accused 30 Greenpeace activists of piracy and say they will prosecute all of them for trying to board an Arctic oil platform.
Russia’s Investigative Committee, modelled on the FBI, will question the activists. Six Britons are among them.
Their ship, the Arctic Sunrise, is being towed to the port of Murmansk.
Committee spokesman Vladimir Markin said “all those who assaulted the platform, regardless of nationality, will be prosecuted”.
The campaigners were detained on Thursday along with their ship after two Greenpeace activists tried to climb onto a Gazprom offshore platform, in a protest against oil drilling in the Arctic.