DONETSK, Ukraine (AP) — Ukrainian troops took control of a checkpoint Tuesday on the western edge of the main rebel stronghold of Donetsk as the government makes further gradual advances to quash pro-Russian separatist forces in the east.
An Associated Press reporter saw a tank waving the blue-and-yellow Ukrainian flag in the suburb of Marinka, which lies on a key road into Donetsk. Rebel combatants could still be seen in sniper positions in a nearby area closer to the city center.
As government troops try to tighten their grip around Donetsk, officials in Kiev are warning of a spike in Russian troop numbers along the eastern border, arousing renewed fears of an incursion by Russia.
The onslaught against separatist fighters has focused recently on besieging the main rebel strongholds of Donetsk and Luhansk, as well as the smaller but strategically important town of Horlivka.
The wild card is whether Russia, which has been building up troops along the nearby border, will come to the rebels' rescue. President Vladimir Putin has faced increasing pressure from Russian nationalists urging him to send in the army to back the insurgency.
After not seeing much fighting other than a rebel attempt in May to seize the city airport, Donetsk has come under shelling in recent weeks. City authorities estimate that around 200,000 people in the city of 1 million have left their homes. The airport is closed, but buses and trains are still running.
Pro-Russian separatists in eastern Ukraine have been battling the Kiev government since April. Ukraine and Western countries have accused Moscow of fueling the mutiny with weapons and soldiers, a claim the Russian government has repeatedly denied.
The West has also accused Russia of most likely providing the insurgents with surface-to-air missiles that may have been used to shoot down a Malaysia Airlines passenger jet over rebel-held territory on July 17, killing all 298 people on board.
Despite the presence of pro-government forces at the western entrance into Donetsk, defense officials said that the situation there remained in flux.
A spokesman for the Ukrainian military operation in the east, Oleksiy Dmitrashkovsky, said further fighting was expected to ensure full control over Marinka.
On Monday, security officials announced that government troops had taken over Yasinuvata, a town just north of Donetsk that is the site of an important railway junction.
Security spokesman Andriy Lysenko said that victory would enable the army to cut off a key rebel weapons supply route.
The government has for several days claimed success in another key tactical goal of driving a wedge between Donetsk and Luhansk. Fighting in towns between those locations has taken a heavy toll on government troops, however.
Around 20 kilometers (12 miles) north of a town that has seen some of the bitterest clashes, a team of more than 100 international investigators from Australia, the Netherlands and Malaysia is traveling daily to the site where Malaysia Airlines Flight 17 was shot down. The recovery and investigation operation was delayed Monday after shooting was heard at the search location.
Lysenko said Tuesday that 45,000 Russian troops equipped with 160 tanks, more than 1,300 armored vehicles, 192 military airplanes and 134 attack helicopters are at the moment stationed along the border. NATO's current estimate is that there about 20,000 Russian troops massed just east of Ukraine's border.
Russia escalated its troop presence along the Ukrainian border earlier this year, but then pulled back large numbers of soldiers in May.
Both the United States and Ukraine say Russia has again built up its military presence since then. Moscow denies that claim.
Ukraine has accused Russia of firing artillery and rockets at Ukrainian forces across the border to back the rebels. Moscow has rejected these allegations.
A small international monitoring mission was last month authorized by Russia to observe a portion of the border that separatists rebels have under their control. The section of border under observation is very small, however, and the monitoring team minimally staffed.
Also Tuesday, New York-based advocacy group Human Rights Watch said rebel forces have worsened the humanitarian crisis provoked by the ongoing conflict in eastern Ukraine by targeting medical workers.
The group said in a statement that separatists had "threatened medical staff, stolen and destroyed medical equipment and hospital furniture, and compromised the ability of civilian patients to receive treatment."
It also said insurgents have commandeered ambulances and used them to carry rebel fighters.
"Civilians are already bearing the brunt of the conflict in eastern Ukraine," HRW researcher Yulia Gorbunova said. "Interfering with the medical assistance they need is simply unconscionable."
Fighting across eastern Ukraine has forced more than 285,000 people to flee their homes, according to U.N. figures released Tuesday.
The U.N. refugee agency said more than 117,000 have left for other places inside Ukraine, while another 168,000 people have crossed into Russia.
Peter Leonard in Kiev contributed to this report.