Malian soldiers patrol in a street in Diabaly on Wednesday.

Story highlights

NEW: Islamist extremists destroy a bridge near Tassiga, in southeastern Mali

NEW: U.N. official warns the crisis has heightened the overall terrorism threat

NEW: More than 150,000 people have fled Mali, while another 230,000 have been internally displaced

Malian security forces have made their deepest gains yet into territory controlled by Islamist militants, taking control of the city of Hombori, a military official told CNN on Friday.

Hombori is 150 miles away from the rebel stronghold of Gao, and is in an area that the militants have controlled for about 10 months. Details of the fighting were not immediately available.

Read more: What’s behind the instability in Mali?

At the same time, Islamist extremists destroyed a bridge near Tassiga, on the country’s border with Niger, a military spokesman told CNN. Tassiga is south of Gao.

Mali’s military offensive against the militants has gathered pace in the past two weeks, with backing from France and other international allies.

Refugees tell harrowing stories of life under the Islamist militants who hold northern Mali in an iron grip.

But the French-based International Federation for Human Rights said it is “very alarmed” by reports that Malian soldiers are themselves carrying out extrajudicial killings and abuses as they counterstrike.

Read more: More signs al Qaeda behind Algeria attack

France has 2,150 soldiers on Malian soil, with 1,000 more troops supporting the operation from elsewhere.

Between 700 and 800 African troops from Benin, Nigeria, Togo and Burkina Faso have arrived, U.S. State Department spokeswoman Victoria Nuland said. Senegalese troops and up to 2,000 from Chad are on the way, she said.

French involvement in the conflict began on January 11, the day after militants said they had seized the city of Konna, east of Diabaly in central Mali, and were poised to advance south toward Bamako. Until 1960, Mali had been under French control.

Ethnic Tuareg rebels of the separatist party MNLA, who had returned to Mali well-armed from fighting for late Libyan leader Moammar Gadhafi, staged a military coup last year against the Malian government.

Read more: ‘Many deaths’ as Malians, French fight militants

Islamic extremists, capitalizing on the chaos, carved out a large haven in Mali’s north and imposed a strict interpretation of Sharia law. The Islamists banned music, smoking, drinking and watching sports on television. They also destroyed historic tombs and shrines.

Those events stoked fear among global security experts that Mali could become a new hub for terrorism.

“As developments unfold in Mali, the risks for infiltration and destabilization are real in some of the countries bordering Mali, as illustrated by the efforts of neighboring countries to tighten security along the borders,” said Said Djinnit, who heads the U.N. Office for West Africa.

He stressed that the crisis is having “far-reaching effects” in West Africa and the Sahel, and that it has heightened the terrorism threat in the subregion.

The effects of the fighting have not been contained to Mali.

According to the latest U.N. estimates, more than 150,000 people have fled the country for Mauritania, Niger and Burkina Faso, while another 230,000 have been internally displaced.

An al Qaeda-linked group that took responsibility for a massive hostage-taking at a natural gas facility in Algeria this month said it did so in retaliation for Algeria allowing France to use its airspace to fight the Islamist militants in Mali.

At least 37 hostages lost their lives when Algerian forces ended the standoff by storming the complex.

The dead included Japanese, Filipino, American, British and Algerian citizens.

Norwegian oil company Statoil on Friday said that two of its employees were among the victims. Tore Bech, 58, and Thomas Snekkevik, 35, were killed, the company said, adding that three of its employees remain missing.