US Secretary of State John Kerry said Saturday that Washington will decide "very soon" on freeing military aid to Egypt frozen since the army toppled Islamist president Mohamed Morsi in 2013.

Kerry made the announcement on the sidelines of an international investment conference in Egypt, after four Arab states pledged to offer $12 billion (11.4 billion) in investment aid to help revive its economy.

"With respect to aid and assistance, I really expect a decision very soon," Kerry told reporters in response to a question on when Washington planned to release the $650 million in military aid it froze after Morsi's overthrow.

Washington annually offers about $1.5 billion in aid to Egypt, including $1.3 billion in military aid.

A part of it was frozen at the height of a deadly crackdown on Morsi's followers after his overthrow and arrest.

Washington says that freeing this aid depends on Egypt progress in democratic reforms.

The United Nations and rights groups accuse President Abdel Fattah al-Sisi, who as then army chief ousted Morsi before being elected, of rights abuses.

Hundreds of Morsi supporters have been killed and thousands imprisoned in a government crackdown, with rights groups accusing Sisi of installing a regime more repressive than the one of longtime strongman Hosni Mubarak.

Mubarak was toppled after an 18-day uprising in early 2011, leading to years of unrest. Islamists revile Sisi, but he is popular among Egyptians who say the country needs a firm hand.

Washington like several Western capitals remains critical of the crackdown, but realises that Sisi, who leads the biggest Arab military force, cannot be ignored in the fight against the Islamic State group.

Sisi has called for a unified Arab military force to fight the jihadists who are also present in the Egypt's Sinai, and he ordered air strikes against the militants in Libya last month.

Washington had released some military aid, including the delivery of Apache helicopters Egypt says are important for its fight against the insurgents in Sinai.

"The US is committed to strengthening the partnership with Egypt," Kerry said.