BEIRUT — The Syrian military opened a second urban front Friday, attacking the rebel stronghold of Homs with the most intense artillery barrage in months and putting opposition fighters there and in Syria's largest city, Aleppo, increasingly on the defensive.
Syria's civil war has been locked in a bloody stalemate, and embattled President Bashar Assad could extend his hold on power if he retakes Aleppo and Homs. Amateur video from Homs, a symbol of resistance, showed black columns of smoke rising from the city, as loud explosions went off every few seconds.
While Assad stepped up attacks at home, tensions with neighboring Turkey flared again Friday, reviving fears that the 18-month-old conflict in Syria could ignite a regional conflagration.
The crisis began on Wednesday, when a Syrian shell killed five civilians in a Turkish border town and triggered unprecedented artillery strikes by Turkey, coupled with warnings that Turkey would no longer tolerate such acts. On Friday, a Syrian mortar round again hit inside Turkey, causing no injuries, and Turkish troops returned fire, the state-run news agency Anadolu said.
In the past, Turkey did not respond to stray Syrian shells, but Turkish Prime Minister Recep Tayyip Erdogan suggested Friday that those days are over. "I once again call on Assad's regime and its supporters: Do not try to test Turkey's patience, do not try to test Turkey's limits," Erdogan said.
Earlier in the day, Turkey had deployed more troops on its border with Syria.
The U.S sided with Turkey, condemning what White House spokesman Josh Earnest called the "aggressive actions of the Syrians." Earnest said Turkey's response was appropriate and that the U.S. stands by Turkey, a NATO ally.
Still, there were signs that both sides are trying to defuse the situation.
Since Wednesday's deadly shelling, Syria has pulled tanks and other military equipment away from the border, a Turkish Foreign Ministry official said, speaking on condition of anonymity in line with government regulations. He said the weaponry was moved far enough to remove the "perception of threat."
Syrian officials could not be reached for comment.
Turkey, along with other countries siding with the rebels, is averse to intervening militarily, while Assad has also tried to avoid provocations he believes would trigger a foreign intervention.
Undeterred by its troubles with Turkey, the Syrian regime on Friday launched a new offensive against Homs, unleashing heavy shelling and air attacks. The attack is the worst Homs has seen in five months, said the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground.
"Around dawn, the regime went crazy and started shelling hysterically," Homs-based activist Abu Rami told The Associated Press via Skype from rebel-held Old Homs. "An average of five rockets a minute are falling," he said, asking to be identified by his nickname for fear of reprisal. Most residents who still live in rebel-held areas around the city were hiding in shelters, he said.
Regime forces fired rockets and mortar shells at the rebel-held neighborhoods of Old Homs, Khaldiya, Qusour and Jouret el-Shayah, the activist said, adding that regime forces were also targeting villages around Homs and the rebel-held town of Rastan to the north.
Earlier this week, Syrian refugees in Lebanon said their villages in Homs province had come under heavy air attack from so-called barrel bombs, makeshift weapons consisting of containers stuffed with explosives.
Mohammed Yousef, a 25-year-old rebel fighter taking a break from battle in the Lebanese border town of Arsal, said most homes in his village, Zar'a, were leveled in the recent air attacks. Yousef said he fled last week, after his home was destroyed.
Homs, Syria's third largest city and a center of the uprising against Assad, was subjected to intense regime shelling in February and March, the first area to suffer widespread devastation. The regime assault slowed in April, as the focus shifted to other areas, including Aleppo, where rebels first seized control of some areas in an offensive in late July.
After weeks of stalemate in Aleppo, rebel fighters announced a new push last week to take the city, but the regime has fought back hard, shelling from tanks and bombing from the air.
An AP journalist in Aleppo said fighting has intensified this week, and that the rebels appear to be losing some ground in close-quarter combat. Over a period of a few days, rebels lost control of several buildings in one of the front-line neighborhoods, Saif al-Dawla, he said.
Syria's conflict began with a peaceful uprising against Assad, inspired by last year's Arab Spring rebellions against authoritarian rulers. Amid an escalating regime crackdown, the rebellion gradually turned into a civil war. The regime's troops are stretched thin, enabling rebels to control large stretches of countryside in Syria's most densely populated west.
But neither side has been able to deliver a decisive blow, even though the Syrian military has superior weapons, including combat aircraft. In recent weeks, rebels have been targeting Syrian aircraft and air bases in hopes of reducing the regime's advantage.
On Friday, amateur video posted by activists showed what appeared to be a Syrian government helicopter hurtling to the ground with a trail of white smoke behind it. Rami Abdul-Rahman, the head of the Observatory, said he was told by rebels that the helicopter was hit over Saqba, a town east of Damascus.
Another video, posted late Thursday, showed the purported capture of an air defense base by rebels. The video showed dozens of gunmen outside an area where smoke was billowing. Off camera, one gunman says a "missile air defense battalion" had been captured. Another video showed missiles inside a room.
Activist Mohammed Saeed, based in the Damascus suburb of Douma, said rebels captured an air defense base in the Eastern Ghouta area near the capital on Monday.
Syria imposes tight restrictions on foreign journalists and the content of amateur videos cannot be confirmed independently.
The rebels did not give any other evidence that would confirm the capture of a base, or identify the location of the video.
If confirmed, the capture of a stock of working anti-aircraft missiles would be a boost to a lightly-armed rebel force that says it faces frequent attacks by low-flying helicopters and warplanes.
Associated Press Writer Suzan Fraser In Ankara, Turkey, contributed to this report.