BENGHAZI, Libya – Forces loyal to Libyan leader Moammar Gadhafi unleashed their fiercest counterattack yet against the opposition Friday, assaulting rebel-held positions by ground and air and firing on demonstrators in the government stronghold of Tripoli.

The lethal force of the government offensive — including what rebels described as a “bloodbath” in the strategic western port city of Zawiyah — raised the stakes for Washington and its Western allies. They have threatened military intervention should the Gadhafi government cross red lines including the systematic endangerment of defenseless civilians or if the battle for Libya evolved into a long-term, bloody stalemate.

Yet if anything, the events Friday underscored Gadhafi’s ability to press defiantly ahead with a brutal campaign to reclaim land already lost to the rebels and squelch dissent within bastions of government control. The government appeared to be attempting to secure a buffer zone around Tripoli and target areas vital to the country’s oil industry, taking aim at cities and ports that have given the rebels a foothold close to the capital.

The White House expressed renewed alarm, saying that President Obama is “appalled by the use of force against unarmed, peaceful civilians.” Obama is being briefed on Libya three times a day, and “we’re not taking any options off the table,” said Jay Carney, the White House press secretary.

With thousands of refugees stuck on the Tunisian border with Libya, the U.S. Air Force flew in humanitarian supplies for them Friday aboard two C-130 cargo planes and planned to return today to pick up Egyptian refugees and fly them home.

The fiercest attack Friday fell on the opposition-held city of Zawiyah, home to one of Libya’s largest oil refineries and situated just 27 miles west of Tripoli. Official Libyan media claimed the government had retaken the city, but the rebels there denied it. As of late Friday, however, the city remained under siege.

“We are still in the square,” said Mohamed Magid, an opposition spokesman. “Zawiyah has not fallen.”

Gadhafi loyalists armed with tanks and heavy machine guns launched an offensive around midday, rebels said. Forces loyal to Gadhafi entered the city from several directions, using tanks, SUVs and trucks armed with heavy machine guns, witnesses said. They also laid siege to the city with mortar fire.

Though details were impossible to verify, witnesses in Zawiyah said at least 15 people were killed and 200 wounded. Some reports put the death toll as high as 50.

One rebel fighter said Gadhafi loyalists shot at people in front of a hospital, blocking the injured from getting treatment. Pro-Gadhafi forces reached the city’s gates, climbing the tallest buildings just outside the edge of town and firing indiscriminately on crowds, witnesses said.

In the capital, meanwhile, Gadhafi again moved swiftly against his opponents, who witnesses described as largely unarmed protesters attempting to demonstrate in various parts of the clamped-down city.

As hundreds of protesters took to the streets after Friday prayers, government forces opened fire with live ammunition, according to eyewitnesses and reports.

In the Bab al-Aziziyah neighborhood of Tripoli, protesters returned to their homes when faced with security forces. But others turned out in the city’s Green Square chanting slogans against the regime, scattering only when bullets started to fly. “I heard the shots,” said a 33-year-old electrical engineer who lives near the square. “I don’t know how many were killed.”