DAKAR, Senegal (AP) — Attackers opened fire on a temporary United Nations base in central Mali on Wednesday in a well-planned assault that wounded 28 peacekeepers from Togo, U.N. officials said.

The United Nations condemned the attack in Mali, which remains “the deadliest” of the U.N.’s 12 far-flung peacekeeping missions, U.N. spokesman Stephane Dujarric said. Just so far this year, five peacekeepers have been killed and 46 injured by hostile acts in the violence-plagued West African nation, he said.

Mali has been in turmoil since a 2012 uprising prompted mutinous soldiers to overthrow the president. The power vacuum that resulted ultimately led to an Islamic insurgency and a French-led war that ousted the jihadists from power in 2013.

But insurgents remain active and extremist groups affiliated with al-Qaida and the Islamic State group have moved from the arid north to more populated central Mali since 2015, stoking animosity and violence between ethnic groups in the region. The country has also been plagued by a series of coups, the latest in August that overthrew President Ibrahim Boubacar.

There have been no claims of responsibility for what the U.N. mission called Wednesday’s “complex” attack using direct and indirect fire against the temporary base in Kerena, near Douentza. But Islamic extremists linked to al-Qaida and the Islamic State group have staged regular attacks on U.N. peacekeepers and soldiers.

Mahamat Saleh Annadfi, the U.N. special representative for Mali and head of the peacekeeping mission, “strongly condemns this cowardly attack on the peacekeepers and has ensured that all measures are taken to ensure that the wounded receive appropriate treatment,” mission spokesman Olivier Salgado said.

The more than 12,500 U.N. peacekeepers and nearly 1,700 international police in Mali work in “a very challenging and hostile environment,” Dujarric said in New York, noting that there are not only extremist groups but “a lot of nebulous armed groups.”

“I think, given the level, the complexity of the attack that we’ve seen, this was something that was clearly well planned,” he told reporters.

For several months now, peacekeepers have been carrying out numerous security operations in central Mali to help reduce violence against civilians and to restore calm in areas where community tensions are reported, Dujarric said. They are also working to reduce the threat of improvised explosive devices, an issue in the Douentza region.

The U.N. spokesman said the key unresolved issue in Mali “is the lack of political progress." He said all Malian leaders must join the political discussions and lay down their arms.

“U.N. peacekeepers are not meant ... to conduct counterterrorism operations on a regular basis,” Dujarric said. “They are peacekeepers. For there to be a peace to keep, we also need political leaders to assume their responsibilities across the board.”

“And it bears reminding that the investigation, the prosecution for these attacks, also is the primary responsibility of the government of Mali, and they also need to investigate it,” the U.N. spokesman said.

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Associated Press writer Edith M. Lederer at the United Nations contributed to this report.