Quoting a Russian Defense Ministry source, RIA Novosti said satellite photos showed the force forming around the city that has become a friction point between the Ukraine military and pro-Russian militants.
The Defense Ministry source said the number of Ukraine troops put the pro-Russian militants at a disadvantage because the latter are "armed only with small amount of pistols and shotguns."
Many eastern Ukraine residents have Russian roots and sympathize with Moscow.
The source said the photos showed about 160 tanks, 230 infantry combat vehicles and armored personnel carriers, mine throwers and multiple-launch rocket systems.
Russian President Vladimir Putin has repeatedly criticized Kiev's use of force against Ukrainian civilians.
Developments in Ukraine have come at a rapid pace in recent days: -- Russia, which already had 40,000 troops on its side of the border, started new military drills a few days ago after Ukrainian forces said they killed five pro-Russian militants.
Ukraine launched the second stage of an "anti-terrorist operation" against militants in Slavyansk.
On Friday, a team of European and Ukrainian military observers were seized Friday by pro-Russian separatists in Slavyansk.
Russian military aircraft "crossed and violated" Ukrainian airspace seven times overnight, Ukrainian Prime Minister Arseniy Yatsenyuk told reporters in Rome on Saturday.
The Russian Defense Ministry denied the accusation, according to the state news agency Itar-Tass.
Yatsenyuk met with Pope Francis while in Rome on Saturday.
The meeting has been seen as a sigh of support from the Vatican for his government.
G7 leaders said they would impose new sanctions on Russia over its role in the crisis.
The Ukrainian Prime Minister urged Russia to pull back its security forces and not to support pro-Russian militants in eastern and southern Ukraine.
"We urge Russia to leave us alone," he said in televised remarks.
Ukraine's government has promised constitutional reforms and protections for Russian speakers in a bid to ease the tensions in its eastern regions.
Inspectors seized in Slavyansk
On Saturday, the fate of the military inspectors preoccupied world leaders.
The inspectors from the Organization for Security and Co-operation in Europe were detained Friday as they entered Slavyansk, along with five Ukrainian military representatives and the driver of their bus, Ukraine's Interior Ministry said.
Ukraine's Security Service, the SBU, said the group is being kept under "inhumane conditions" in the basement of a building held by the militants.
The self-declared mayor of Slavyansk, Vyacheslav Ponomarev, told reporters that one of the "prisoners" has diabetes, but he has the medicine he needs and will be given his own quarters overnight.
Separatist leader Denis Pushilin, self-declared chairman of the so-called "Donetsk People's Republic," told CNN he doesn't believe they are from the OSCE, but that some are NATO spies.
The German Foreign Office said it had set up an emergency task force to find out what has happened to the team members, four of whom are German.
The others are from Denmark, Poland, Bulgaria and the Czech Republic, Russian state media said.
The OSCE mission in Ukraine is tasked with helping to implement an international agreement signed nine days ago in Switzerland, which called for illegal militia groups to disarm and leave occupied buildings, among other provisions.
In a phone call with U.S. Secretary of State John Kerry, Russian Foreign Minister Sergey Lavrov asked the United States to use its influence to secure the release of pro-Russian leaders being held in Ukraine.
Kerry urged Russia to support efforts of the OSCE and the government of Ukraine to liberate the inspectors and their Ukrainian guides, according to a senior State Department official.
Against the backdrop of increasing volatility in Ukraine, leaders of the G7 industrialized nations on Friday announced they would "move swiftly to impose additional sanctions on Russia" over its actions in Ukraine.
The statement from the group -- which includes Canada, France, Germany, Italy, Japan, the United Kingdom and the United States -- came hours after U.S. President Barack Obama threatened Russia with new sanctions.
Source: CNN World