Top Ukrainian officials have insinuated that Russian security services had a hand in the violence that led to more than 100 deaths in Kiev last month, as Moscow claimed 25 Ukrainians had planned terror attacks in Russia last month and had been detained.

At a press conference in Kiev, Arsen Avakov, Ukraine's interim interior minister, claimed his predecessor Vitali Zakharchenko, who is currently on the run, was directly involved in giving orders to shoot at protesters, along with the SBU security services. He said a dozen Ukrainian police officers had been detained on suspicion of carrying out the shooting.

Valentyn Nalivaichenko, the new head of the SBU, added that a number of officers from Russia's FSB had been consulting with the SBU in Kiev in December and January, and that Russian citizens were present at SBU headquarters.

He also claimed that explosives and weapons were delivered to Ukraine from Russia during the protest period, including on the day of the bloodiest violence, 20 February. There was no information given about the sources for the information, nor any evidence provided to the public to back up the allegations. The FSB's press service said: "Let these statements remain on the conscience of the SBU."

The ousted Ukrainian president, Viktor Yanukovych, denied any involvement in ordering the sniper attacks. Yanukovych fled Kiev shortly after the shootings and eventually left Ukraine for Russia, where he now lives.

He still claims to be the legitimate president of Ukraine, although even his own party has disowned him. He confirmed that he had asked Russia to send troops to Crimea, a decision he now says was a mistake, after Moscow moved to annex the territory last month.

The Kremlin has put forward a different version of events, alleging the violence that spurred Yanukovych's downfall was organised by far-right radical protest groups keen to radicalise the situation. On Sunday, Russia's foreign minister, Sergei Lavrov, said Moscow had evidence that Ukrainian nationalist groups were behind the sniper attacks.

In an interview with the Russian media, he said that while he could not be 100% certain, "many factors point to this". Lavrov said he had repeatedly raised the allegations with western partners and hoped they would not be swept under the carpet. There is also footage that purportedly shows snipers shooting at protesters and riot police, which Russia says is evidence that the violence was carried out by "provocateurs".

A recording of a phone call between the EU foreign affairs chief, Catherine Ashton, and Estonia's foreign minister, Urmas Paet, was leaked last month – with suspicions falling on Russian intelligence or organisations close to it – in which the pair discussed the theory that the opposition itself was responsible for the snipers.

Paet tells Ashton he has heard discussions in Kiev in which people claim possible involvement, to which she responds with surprise. Russian media have seized on the call as evidence that there has been a western cover-up about supposed involvement.

On Thursday, Ukrainian officials said their investigation had determined some, but not all, of the sniper locations, and that 12 people had been arrested after thousands of riot police were questioned. Officials said some of the Berkut (special police) officers whom Kiev wants to arrest were in Crimea. Hundreds of Berkut returned to the peninsula after the violence in Kiev and were treated as heroes.

After the new government disbanded the regiment, Russia said it would give Berkut officers passports and integrate them into Russian security services, while former Berkut officers played a part in the Crimean "self-defence" forces that sprang up before the Russian moves to annex the territory.

On Thursday evening, a Russian television channel showed footage of what it said were Ukrainian radicals captured last month, who it said had planned to carry out terrorist acts in various regions of Russia around 16 March, the day of Crimea's referendum. The channel said three of the men were members of Right Sector, the nationalist group Lavrov announced was behind the sniper attacks.

Also on Thursday, the head of Russia's gas behemoth Gazprom said Ukraine would now have to pay $485 per 1,000 cubic metres of gas because of a new export duty. The increase came just two days after Gazprom announced a 44% rise in the price to $385.50 and will increase financial pressure on Ukraine's troubled economy. Analysts say Ukraine will struggle to pay the new price.