Egypt and Sudan held joint military drills in Sudan this week amid reports that Ethiopia had begun a second filling of its controversial Nile dam.

Egyptian ground, naval and air forces, including special forces and paratroopers, participated in the Guardians of the Nile exercise, which kicked off Thursday, an Egyptian military spokesman said

Sudanese media announced the exercise last week, saying that Egyptian soldiers had arrived at Khartoum Air Base and others were expected to arrive by ship. The exercise is expected to continue through the weekend, and is aimed at strengthening ties and “unifying methods to deal with threats that both countries expectedly face,” according to Sudan’s state-run SUNA news agency.

The two countries previously combined forces for training drills in April. Those exercises took place after negotiations aimed at easing tensions with Addis Ababa fell through in Kinshasa.

Cairo and Khartoum first began joint military exercises in November in response to what they perceive as a refusal by Ethiopian Prime Minister Ahmed Abiy’s government to commit to written guarantees on the dam’s usage.

Egypt in particular fears the Grand Ethiopian Renaissance Dam could severely reduce its access to Nile water, potentially threatening the livelihoods — and lives — of millions of people. Egypt already recycles more fresh water than any other country in Africa to support its agriculture and 100 million people.

Egyptian President Abdel Fattah al-Sisi issued a hedged warning  following the collapse of the Kinshasa talks in April, saying, “I tell our Ethiopian brothers: ‘We should not reach the level that you mess with a water drop in Egypt, because all options are open.’”

“Cooperation between each other and building together is much better than when we disagree and struggle,” Sisi added at the time.

Egypt has been building defense and intelligence ties with east African countries as more than a decade of talks with Ethiopia over the dam continue to falter. A military delegation from Cairo signed a defense agreement with officials in Kenya on Wednesday, following agreements with Burundi and Uganda earlier this year.

The US special envoy for the Horn of Africa region, Ambassador Jeffrey Feltman, is returning to the region amid broader tensions that threaten to destabilize the area, the State Department said Wednesday.

Ethiopia’s border dispute with Sudan, and concerns over Addis Ababa’s continued violence in the Tigray region, have US officials concerned that a regional conflict could result, though there is some skepticism in Washington as to Egypt’s desire to engage in conflict over the dam.