Pro-russian troops ride on a tank on a road in the direction of Donetsk not far from Debaltseve, Donetsk region, eastern Ukraine, on June 20, 2014. (Evgeniy Maloletka/AP)

Russia has redeployed more troops to the Ukrainian border, including special forces units that are arming and training pro-Russian Ukrainian separatists, U.S. officials said Friday, as fighting continued to flare in the region.

In Kiev, Ukrainian President Petro Poroshenko said Friday that a week-long, unilateral cease-fire would begin immediately, to give pro-Russian separatists a chance to surrender their weapons and stop fighting.

Tanks and heavy artillery departed from sites in southwestern Russia and headed toward the border over the past 24 hours, according to a senior U.S. official. The Ukrainian government has said that Russian tanks have entered eastern Ukraine, but those reports have not been confirmed by the United States, said the official, who spoke on the condition of anonymity because she was not authorized to speak for attribution.

The official also indicated that Russian weaponry flowing into Ukraine includes decommissioned equipment no longer in use by the Russian military but still in use by Ukrainian forces — a tactic that could be meant to disguise the equipment as Ukrainian. U.S. officials have also said that a multiple-launch rocket system belonging to Russian forces in Chechnya is now in Ukraine.

Russia denies any buildup, however, and characterized any troop movement near the border as an attempt to strengthen its own defenses as a war rages just across its border.

The reported movements came as the Treasury Department announced new sanctions on seven separatists said to be responsible for unrest in eastern Ukraine, including Igor Girkin, a leader of the self-declared Donetsk People’s Republic.

“The United States will continue to take action to hold accountable those persons engaged in efforts to destabilize Crimea and eastern Ukraine,” David S. Cohen, the Treasury’s undersecretary for terrorism and financial intelligence, said in a statement. “These individuals have all contributed to attempts to illegally undermine the legitimate government in Kyiv, notably by falsely proclaiming leadership positions and fomenting violent unrest.”

The sanctions, which freeze any assets the targets have in the United States and prohibit U.S. firms or businesses from dealing with them, are the latest round of economic measures to be imposed on separatist leaders and Russian officials since the unrest in the region began.

News of renewed Russian involvement on Ukraine’s border follows attempts by Ukraine’s new president to implement a peace plan to end the two-month-old conflict with separatists in the eastern part of the country. The plan includes a proposal to create a six-mile buffer zone on Ukraine’s border with Russia, according to details that started to emerge Friday.

Poroshenko, who discussed his plan late Thursday in a phone call with Russian President Vladimir Putin, said in a statement posted on his presidential Web site that “terrorists can lay down their arms, and those who will not do this will be killed.”

Ukraine declared the unilateral cease-fire Friday evening. The temporary truce will stay in place for seven days, a period that would allow pro-Russian separatists to disarm and stop fighting. Poroshenko offered amnesty to fighters who did not commit major crimes and safe passage out of the country for “mercenaries.”

Though many details of his peace plan were already known, Poroshenko unveiled more in a 15-item list that is a combination of proposals to end the fighting and programs to eventually expand regional power, add jobs and repair buildings damaged in the fighting.

His offers of amnesty and safe passage are bookended by a series of demands. He said people held as hostages must be released, and buildings occupied by the self-proclaimed “people’s republics” in the Donetsk and Luhansk regions returned to government control. He called for the creation of a 6-mile buffer zone along the border from which “illegal armed groups” would be barred.

Poroshenko’s plan also addressed some of the key factors that have alienated many residents of the Russian-speaking east. He proposed amendments to the constitution that would decentralize power and protect the Russian language, even though Ukrainian is the only official language of the country.

The rebels have so far turned a cold shoulder to Poroshenko’s gestures, rejecting his cease-fire offer and saying that they will disarm only when government troops withdraw from the region.

Although government troops will cease offensive operations during the week-long truce, Poroshenko said they will defend themselves and Ukrainian territory.

“It does not mean we will not give resistance in case of aggression,” he said. “We will do everything to protect our territory.”

A few more details of Poroshenko’s plan were published Friday in the Ukrainian media.

From Ukraine’s perspective, the first order of business is to seal off the border with Russia to sever the rebels’ supply lines of fighters and equipment.

Oleksandr Turchynov, the speaker of parliament, said Friday morning that government troops have regained full control of the border. But Vladimir Chepovoy, a spokesman for Ukraine’s National Security Council, said, “There is no such thing yet.”

So far, rebels have rejected every offer to stop fighting. At daybreak Friday, militants launched an attack on government troops around the Luhansk airport, where they shot down a military transport plane last weekend. The government claims it has killed about 300 insurgents in fighting since Thursday morning, losing seven of its own troops.

Ukrainian security officials also said Ukraine has detained 13 Russian citizens among more than 90 “terrorists and saboteurs” in their custody, using a term that appeared to refer to intelligence agents as opposed to fighters.

Given the facts on the battleground, Poroshenko’s peace plan reads more like a blueprint for negotiations than a plan to quickly stop the fighting. Poroshenko has pointedly said he will not sit down to negotiate with “terrorists.” His representative in the negotiations, Irina Gerashchenko, said Friday that separatists of the self-proclaimed “people’s republics” in Luhansk and Donetsk will not be invited to any roundtable talks.

Ukrainian and NATO officials have said that Russia is deploying more troops closer to the border, though there is no indication that Russia intends to respond to a rebel leader’s call to invade and help them survive a punishing onslaught by the Ukrainian army. Moscow may be signaling that its interests need to be taken into account before the crisis can end.

Morello reported from Dnipropetrovsk, Ukraine.