Crimea has been left without power after several pylons supplying electricity from the Ukraine were "blown up" up overnight.
Four power lines were reportedly toppled, leaving two million inhabitants on the disputed Crimean peninsula without electricity.
Russia has declared a state of emergency in Crimea until power is fully restored, news agency RIA reported.
It was not immediately clear who had damaged the pylons, but a Russian senator described the move as an "act of terrorism" and implied that Ukrainian nationalists were to blame.
Ukrainian authorities have said activists tried to block efforts to repair the pylon, and that the damage seen is likely a result of shelling of explosive devices.
Russian President Vladimir Putin ordered for work to start on taking control of Crimea weeks before a referendum which Kremlin officials claimed prompted the region's annexation from Ukraine.
In an interview with Russian state television channel Rossiya-1 broadcast on Sunday, Putin said that the plans were first discussed in February last year during an emergency meeting about the overthrow of Ukrainian president Viktor Yanukovich.
His account is at odds with previous Kremlin statements that claimed that the annexation decision was taken after the referendum on March 16, when Crimeans voted to become part of the Russian Federation.
This was on the night of Feb. 22 through to Feb. 23. We finished around 7 in the morning. And, while saying goodbye, I told all the colleagues: 'We have to start the work on Crimea's return into Russia'."
Western governments have condemned Russia's intervention in Crimea as illegal, with the European Union and United States imposing sanctions on Moscow.
Ukraine's new president Petro Poroshenko has called for dialogue with the east of the country, gripped with a violent, pro-Russian insurgency, as he took the oath of office.
Calling for armed groups to lay down their weapons, he promised Ukraine's parliament "I will bring you peace" but did not indicate whether forces would scale back their offences against the insurgency.
As president, Mr Poroshenko is commander-in-chief of the military and appoints the defence and foreign ministers.
In his inaugural address, attended by US vice president Joseph Biden and senator John McCain, Mr Poroshenko promised amnesty for those without "blood on their hands". He said:
I am calling on everyone who has taken arms in their hands - please lay down your arms.
Ukrainian President Petro Poroshenko has said that "Crimea was, is, and will be Ukrainian" but stated he does not want war or revenge, during his inaugural speech.
He said: "I don't want war; I don't want revenge. I want peace and I want peace to happen," Poroshenko told parliament after taking the oath.
In his inaugural speech he said that he would offer a safe corridor for Russian fighters to go home and that he would "guarantee immunity to all."
He said: "Please, lay down the guns and I guarantee immunity to all those who don't have bloodshed on their hands."
Ukraine's Petro Poroshenko has been sworn in as president of his troubled country as government forces continued to battle pro-Russian separatists in the east.
He will be Ukraine's fifth president since independence in 1991.
During his inauguration ceremony in parliament, he took an oath to protect Ukraine's sovereignty and independence and to safeguard the rights and freedoms of its citizens.
Poroshenko, who won a landslide victory after campaigning on the slogan "Live in a new way", is expected lay out a programme for restoring stability and moving Ukraine into the European mainstream.
Petro Poroshenko is to be sworn in as president of Ukraine, after his victory in the May 25 poll.
Yesterday, he met with Russian President Putin briefly at D-Day commemorations, saying it could be the start of dialogue between the countries.
The Obama administration has attacked Vladimir Putin over his visit to Crimea today, calling the move "provocative and unnecessary".
The Russian President visited the peninsula, annexed from Ukraine in March, to mark the anniversary of the Soviet Union's victory over Nazi Germany.
US State Department spokeswoman Jen Psaki issued a strong condemnation of the visit, saying:
"This trip is provocative and unnecessary. Crimea belongs to Ukraine and we don't recognize, of course, the illegal and illegitimate steps by Russia in that regard,"
European Ambassadors have agreed in principle to add approximately 15 people and five Crimean-based companies to the Russia sanctions list, diplomats told Reuters.
President Vladimir Putin has hailed the Crimea region joining Russia as he visited the area for the first time since it was annexed from Ukraine in March.
Putin delivered a speech in the city of Sevastopol and watched a parade of Russian navy ships and a flyby of Russian aircraft which marked Victory Day - the World War Two defeat of Nazi Germany.
Ukraine has condemned Putin's visit as trampling on international law and deliberately escalating the crisis.
Russian President Vladimir Putin has attended a Victory Day parade in Sevastopol, Crimea, marking his first visit to the area since it was annexed in March.
His visit to the region is likely to outrage the West and has already been denounced by the Ukrainian government, with the foreign ministry calling it a "deliberate escalation" of the crisis.
Putin flew to Crimea after attending a military parade in Moscow, which was making the anniversary of the World War Two victory over Nazi Germany.