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DONETSK, Ukraine -- Fighting in eastern Ukraine reached the main rebel stronghold of Donetsk on Tuesday as separatists and government troops exchanged rocket fire in neighborhoods on the edge of the city.
In a western district of Donetsk, rockets apparently fired from a government position landed in the courtyard of a one-story house, killing an elderly resident.
The spread of fighting into the city itself marks a decisive turn for the conflict, but one that will arouse fears of an escalation in the humanitarian crisis gripping the country.
As government troops try to tighten their encirclement of rebel citadels, officials in Kiev have warned of a spike in Russian troop numbers along the border, arousing renewed fears of intervention by Moscow.
In the morning, an Associated Press journalist saw a tank bearing the Ukrainian blue-and-yellow flag at a key checkpoint in the western suburb of Marinka, which lies on a road into Donetsk.
Rebel combatants could still be seen hunkered down in sniper positions in a nearby area closer to the center.
Defense officials swiftly played down the presence of government troops in Marinka, insisting that more fighting lay ahead before the area could be properly secured.
By the afternoon, however, there were signs the army was looking to advance further into the city.
The shell that landed by the house in western Donetsk's Petrovsky district killed a resident that neighbors identified only as Galina, along with her dog.
An AP journalist saw Galina's body as it was covered by a plastic sheet and carried from her house by rescue workers.
Her next-door neighbor, Sergei Lozinsky, 47, said he was in his house when the blast hit.
"I fell to the ground and covered my head with my arms when the shell fell. It all happened so fast. And my windows were all blown out," he said.
Another neighbor, 32-year old teacher Nataliya Subbota, said Galina had until recently taken refuge in bomb shelters.
"We hid in basements night and day. The shelling would get quiet and then start again. But every day they became more intense," Subbota said, before referring to Galina by a nickname. "Granny Galya got sick of hiding with us in the basement. And now look what has happened."
Separatist fighters responded to incoming fire with rocket attacks of their own.
The sound of return mortar shell fire and the distinctive multiple whooshes of Grad rocket launchers could be heard from areas surrounding the neighborhood targeted by army forces. Mortar shells appeared to have been fired from inside residential areas.
One rebel fighter, who gave his name as Sanya Pskovsky, said he was kept busy all day fighting back the government onslaught.
"I sign every mortar. They get names. Only today I sent 15 mortars. Little gifts," he told the AP.
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.
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.
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, and Mstyslav Chernov in Donetsk, contributed to this report.