Djibouti has accused neighboring Eritrea of occupying a disputed territory along the two neighboring countries' borderline.
"Qatari peacekeepers withdrew on June 12 and 13. On the same day, there were Eritrean military movements on the mountain," Foreign Minister Mahamoud Ali Youssouf said on Friday.
Qatari peacekeepers had been in control of the disputed mountainous border crossing between Eritrea and Djibouti.
"They [Eritreans] are now in full control of Dumeira Mountain and Dumeira Island. This is in breach of the United Nations Security Council resolution," he said.
The minister said Djibouti had lodged complaints with both the United Nations and the African Union, adding that its military forces were "on alert".
On the other side of the dispute, Nasredin Ali, a spokesman for Eritrea's biggest armed group, known as the Red Sea Afar Democratic Organization, confirmed that Eritrean forces moved into the disputed region after the Qatari troops pulled out.
Qatari forces were deployed to the region after deadly clashes broke out between the Horn of Africa countries in 2008. The UN Security Council then requested both sides to withdraw from the area, before the neighbors accepted a Qatari request to mediate and deploy peacekeepers.
Qatari peacekeepers monitoring the disputed territory were summoned home days ago after Djibouti and Eritrea both joined the Saudi-led isolation of Qatar.
On June 5, the governments of Saudi Arabia, the United Arab Emirates, Bahrain and Egypt announced they were severing ties with Qatar.
In addition to them, Yemen, the eastern government of Libya, the Maldives, Mauritania, Senegal and Eritrea announced they were cutting ties with Qatar, as well.
Jordan and Djibouti also announced they were downgrading their diplomatic representation with Qatar.
These countries accuse Qatar of supporting terrorism, which Doha strongly denies.
Qatar has slammed a blockade after the ties were cut as part of a “policy of domination and control” led by the Saudi "big brother".