The charge comes a day after Russian President Vladimir Putin called for Kiev to hold unconditional talks with rebel leaders, saying it was the only way that a peace plan offered by Ukrainian President Petro Poroshenko could be successful.
Mr. Poroshenko has said he is willing to talk with anyone who has opposed the government except for those with "blood on their hands," effectively ruling out much of the rebel leadership.
"In response to this offer of peace, the militants and terrorists have continued their armed attacks, blatantly violating the truce," Ukraine's defense ministry said in a statement.
Western leaders are threatening broader sanctions against Russia if Mr. Putin doesn't move to stop the separatists from fighting and secure stretches of the border under rebel control.
Kiev has accused Moscow of allowing heavy weaponry and fighters to flood into Ukraine.
EU heads of government will meet at a summit later this week and are set to decide whether the bloc should increase economic pressure on Russia.
"There is an opportunity now for everybody in eastern Ukraine if they support the plan put forward by President Poroshenko," said U.K. Foreign Secretary William Hague on Monday.
"And in the absence of that, the European Union will be able to take further measures, further sanctions on Russia. Those measures are ready…so we now look to Russia to support the peace plan."
While Mr. Putin has come out in support of the peace plan, he has given no sign that he intends to rein in the rebel fighters.
Ukraine's defense ministry said Russia needed to do more for the peace plan to work.
"Our country, along with the entire international community expect actual active support from the Russian Federation for the peace plan offered by the president of Ukraine," the statement said.
On Monday, however, the first signs emerged of a possible framework for future talks when two Ukrainian political figures—one of whom is close to Mr. Putin—served as intermediaries for the separatists at a meeting in Donetsk with Heidi Tagliavini, head of the Organization for Security and Co-Operation in Europe's mission in Ukraine.
Viktor Medvedchuk, a Ukrainian politician known for his close ties to the Kremlin, and Nestor Shufrych, a member of the political party of recently-ousted Ukrainian president Viktor Yanukovych, attended the meeting together with Mikhail Zubarov, Russia's ambassador to Kiev, and former Ukrainian president, Leonid Kuchma, who was there representing Mr. Poroshenko.
The defense ministry said rebels on Sunday shelled 11 separate government positions in the regions of Donetsk and Luhansk, where an insurgency has raged for more than two months, with attacks continuing on Monday.
The state border service also reported several clashes at border posts on both Saturday and Sunday.
A spokesman for the government's military operation against the separatists said Monday that six convoys and government roadblocks in the past day were ambushed by militants firing mortars and sniper rifles, wounding five soldiers and one border guard.
Rebel leaders, however, say the government's forces have failed to abide by the weeklong cease fire.
"Poroshenko's so-called peace plan has not been carried out and the Ukrainian army has actively been fighting for three days," Alexander Borodai, the self-proclaimed prime minister of the Donetsk People's Republic was quoted as saying by the Interfax news agency.
"There is no truce."
Mr. Poroshenko has stressed that despite the cease-fire, Ukrainian forces would return fire if fired upon.
Rebel leaders have said they would not lay down their weapons unless the government forces withdraw completely from the regions under separatist control.
Meanwhile, Russia's finance ministry said Monday it hasn't yet received a coupon payment on a Eurobond that Kiev said had been sent last week.
The Ukrainian finance ministry said Friday it had paid a $73 million coupon on a $3 billion Eurobond purchased by Russia as part of the bailout deal struck between Russia's President Vladimir Putin and his now-ousted counterpart Viktor Yanukovych in late 2013.
"Unfortunately we can't confirm yet receiving a coupon payment on the Ukrainian Eurobond," the ministry's spokesman said.
Ukraine, however, has an option to pay the coupon within the next 10 days as the Eurobond deal includes a grace period.
The head of Ukraine's state-owned gas company, Naftogaz, said Monday he expects talks with Russia and the European Commission to resume in July over the resumption of gas deliveries to Ukraine from Russia.
Russian state gas giant OAO Gazprom cut off supplies to Ukraine on June 16, after demanding Kiev pay back billions in arrears.
Ukraine has refused to pay after Russia raised the gas price by 80% in April, from $268 per 1,000 cubic meters to $485.5 per 1,000 cubic meters, following the ouster of a pro-Russian president in February by protesters.
Since Russia cut off gas supplies, Ukraine has managed to pump between 20 million and 34 million cubic meters of gas a day into underground storage containers through the country's own domestic supplies and from reverse flows from Europe, Naftogaz Chief Executive Andriy Kobolev said.
While Russia has promised that stopping deliveries to Ukraine won't affect supplies of gas to Europe, which transit through Ukraine, Moscow has warned Kiev could siphon off gas for its own use.
The EU has also raised concerns that a fall in the amount of gas held in Ukraine's underground storage facilities could cause damage to the pipeline system.
Source: The Wall Street Journal