Ethiopia began the second phase of filling its Grand Renaissance Dam in early May by holding back water to fill its reservoir, a senior Sudanese official said on Tuesday. The main phase of the second filling of the controversial dam is expected in July and August after seasonal rains fall in Ethiopia, the official at Sudan's irrigation ministry told Reuters. The 6,450 MW hydropower project is being built on the Blue Nile, the Nile River’s main tributary, just 30km from Sudan's border. Ethiopia, Egypt and Sudan have been locked in talks over the operation of the dam for a decade, with the downstream nations fearing water scarcity and even risk of flooding if a solid agreement is not reached. The latest round of talks stalled in April, and fiery rhetoric has escalated the crisis in the weeks since. But Ethiopia has said it will push ahead with a second phase of filling in July and August, even if no agreement is reached. Ethiopian officials have made no comment on the claims that the dam is being filled. The claim from Sudan came as Egyptian military forces arrived in the Sudanese capital of Khartoum ahead of a joint military drill. Sudanese and Egyptian forces will hold the manoeuvres dubbed “Guardians of the Nile” from mid-next week to the end of the month aimed at “strengthening bilateral relations and unifying methods on dealing with threats that both countries are expected to face," Sudan's state news agency SUNA reported on Tuesday. The report did not say how many troops would participate. Apart from those that landed at Khartoum Air Base, another contingent of soldiers and army vehicles were expected to arrive by sea. In March, Egypt’s President Abdel Fattah El Sisi warned his country’s share of the Nile waters was “untouchable” and that there would be “instability that no one can imagine” in the region if Ethiopia fills the reservoir without an international agreement. Cairo and Khartoum have called for the US, UN, and the European Union to help reach a legally binding deal after African Union involvement in the process failed to reach a deal. The agreement would spell out how the dam is operated and filled, based on international law and norms governing cross-border rivers. Egypt relies on the Nile for more than 90 per cent of its water supplies. Ethiopia says the $5 billion dam is essential, and that the vast majority of its population lacks electricity.