NAIROBI, Kenya – More than 50 African Union peacekeepers have died in fighting in Somalia since an offensive against Islamist militants began two weeks ago, officials told The Associated Press on Friday.

The death toll is far higher than any publicly acknowledged casualty figures for the AU, which appears to be trying to keep the extent of its losses under wraps due to political considerations in Burundi, one of two nations providing the bulk of the forces that are fighting alongside Somali troops.

The offensive aims to break the Islamists’ lock on large swaths of the country’s southern and central regions. Al-Shabab, a Somali militant group with links to al-Qaida, has boxed in the government to just a few city blocks of the seaside capital of Mogadishu. The group has instituted a Taliban-style system of rule, with strict edicts enforced by their courts and public executions.

The government has been promising war against the militants for years, but coordination among its poorly trained, seldom-paid forces has delayed that push.

The AU force, known as AMISOM, has publicly confirmed only a handful of deaths since heavy fighting started Feb. 19. An AU spokesman in Nairobi did not answer calls Friday. Burundi’s government spokesman was unavailable for comment.

Wafula Wamunyinyi, the second-highest ranking official on the AU’s commission for Somalia, declined to discuss casualty figures when reached Friday.

Two Nairobi-based diplomats said at least 43 Burundian and 10 Ugandan troops have been killed since Feb. 18, citing information from people involved in the operation. The two spoke on condition of anonymity because they were not authorized to speak to the media.

A Burundian soldier has been captured by militants, and his image and a recorded statement have been circulating on websites used by al-Shabab.

AMISOM says hundreds of militants have been killed in the offensive. AMISOM officials say peacekeepers have taken back insurgent-controlled areas of Mogadishu. The AU says it controls up to 60 percent of the city.

There are about 8,000 AU forces in Mogadishu, with another 4,000 due to begin arriving in the next few months. Almost all are Ugandan or Burundian. They support the country’s weak U.N.-backed government.

Few officials seem willing to acknowledge the heavy casualties AMISOM troops have suffered. But a statement from France’s Ministry of Foreign and European Affairs on Tuesday said that France evacuated 13 AMISOM troops by military aircraft to neighboring Djibouti for medical care.