DODOMA, Tanzania (AP) — For the first time, Tanzania’s government has acknowledged an attack inside the country by Islamic extremists based in neighboring Mozambique.

The attack claimed by an affiliate of the Islamic State group last week marks an ominous expansion of the shadowy group that has killed more than 1,500 people in northern Mozambique since it emerged in 2017.

Tanzania’s inspector general of police, Simon Sirro, told local media on Thursday that some 300 attackers were involved in the assault on Kitaya, a riverside border village in the Mtwara region. He said some were arrested. He did not say how many people were killed.

The growing extremist violence in northern Mozambique has displaced more than 300,000 people, creating an urgent humanitarian crisis, the U.N. World Food Program said last month.

The extremists have dramatically stepped up attacks this year, capturing the strategic port city of Mocimboa da Praia in August. Mozambique’s northern Cabo Delgado province has massive deposits of liquified natural gas, and the violence threatens to disrupt the billions of dollars of international investment.

The extremists risk giving Mozambique “the type of threat that Boko Haram has become in Nigeria,” U.S. Assistant Secretary of State for African Affairs Tibor Nagy told journalists earlier this year.

Tanzania’s confirmation of the attack comes less than a week before the presidential election in which President John Magufuli seeks a second five-year term. He faces criticism by opposition figures and human rights groups that his administration has stifled dissent and even some information deemed to put the East African country in a bad light.