KUNDUZ, Afghanistan (Reuters) - Afghan forces fought to regain control of the northern city of Kunduz on Monday after Taliban fighters pushed into the center of the provincial capital they had briefly captured almost exactly a year ago in their biggest success in 15 years of war.

With fighting also intensifying in the southern province of Helmand, the attack on Kunduz, a day before a major international donors' conference in Brussels, underlined Afghanistan's precarious security situation and the Taliban's ability to strike important targets.

By late evening, officials said government forces had taken back a central square that the insurgents had occupied during the day but fighting was still going on in areas near the police headquarters, the governor's compound and National Directorate of Security headquarters.

Brigadier General Charles Cleveland, spokesman for the NATO-led Resolute Support mission in Kabul, said the situation in Kunduz was "fluid" and U.S. forces were ready to assist.

"Our Afghan partners are responding to the increased Taliban activity within the area, and U.S. forces have multiple assets and enablers in the area to provide support," he said in an emailed statement.

Footage posted by the Taliban on social media appeared to show fighters in Kunduz walking around empty streets, describing how they had captured army strongholds and taken prisoners. Reuters could not verify the authenticity of the footage.

The insurgents issued a statement saying they were protecting the lives, property and honor of residents and working to restore the city to normality.

The militants appeared to have slipped through a defensive security line set up around Kunduz, entering the city itself from four directions before fighting broke out, witnesses said.

As the clashes spread, senior officials, including the provincial governor and the police chief, abandoned the city for the airport.

A year ago to the day, Afghan troops backed by U.S. air strikes and special forces were battling to drive out Taliban militants who had overrun the city.

But there were varying reports about the intensity of Monday's fighting. Although Afghan authorities rushed in special forces and carried out air strikes, some coalition officials suggested the episode was more a raid aimed at spreading panic than a serious assault.

"This is largely something we've seen before," Pentagon spokesman Captain Jeff Davis told reporters in Washington.

"We see the Taliban go into these city centers, do a Western-movie style shoot-them-up, do some raiding, do some looting, raise a flag, and just as quickly as they do that, they are beaten right back out again," he said.

The renewed attack on Kunduz forced officials to cancel a ceremony planned for Monday to commemorate the first anniversary of a U.S. air strike that destroyed a hospital run by the international charity Médecins Sans Frontières.

ATTACKS ACROSS AFGHANISTAN

During the day, Taliban fighters had stepped up attacks in different parts of Afghanistan, including in Helmand, where they are threatening the provincial capital of Lashkar Gah.

Positioned just across the Helmand river from the city center, they took control of Nawa district to the south, killing a district police chief, officials said.

Heavy fighting also continued along the main road to Tarin Kot, the provincial capital of Uruzgan, also in the south, where a Taliban raid on Sept. 8 sparked fears of another collapse like that in Kunduz last year.

The raid on Tarin Kot was beaten back, but it alarmed security officials because the militants were able to enter the city without significant resistance after police abandoned dozens of checkpoints.

The fall of Kunduz last year was one of the most serious blows to the Western-backed government since the withdrawal of most international troops in 2014.

Although the insurgents abandoned Kunduz after a few days, the capture of a provincial capital underlined their growing strength and exposed weaknesses in the Afghan security forces, which control no more than two-thirds of the country. The city has remained effectively besieged ever since.

"Every day the militants come to the city and are pushed back by security forces," provincial council member, Amruddin Wali, said as he stood with security forces on the edges of the city. "There is killing and fighting every day."

Separately on Monday, at least six people were killed and 35 wounded in the northern province of Jawzjan by a bomb in a crowded marketplace. There was no immediate word on who was responsible.

Efforts at reviving peace talks have failed to produce results, but Afghanistan's international partners are expected to agree to maintain billions of dollars in funding for the government over the next four years at the two-day meeting in Brussels.

(Reporting by Afghanistan bureau; Additional reporting by Idrees Ali in Washington; Writing by James Mackenzie; Editing by Louise Ireland)