A South African legal NGO is "strongly considering" bringing contempt of court charges against government officials for allowing President Omar al-Bashir to leave the country on Monday.

The Southern African Litigation Centre (SALC), the group which secured a court order for Bashir's arrest, said in a statement after his flight from South Africa that it would decide on action after the government filed a court-ordered affidavit explaining why it failed to prevent Bashir from leaving.

On Sunday a court ordered a wide range of government ministers and departments, including those responsible for controlling immigration posts and providing security for Bashir, not to allow him to leave the country while the court heard the application for his arrest.

By the time the court granted a detention order on Monday, Bashir had left, with the apparent collusion of South African officials.

In the SALC statement, its director, Kaajal Ramjathan-Keogh, said that "the rule of law ... is only as strong as the government which enforces it."

She said the Department of Home Affairs, which controls border posts, had "allowed a fugitive from justice to slip through its fingers, compounding the suffering of the victims of these grave crimes."

The department's officials have on a number of previous occasions defied court orders preventing individuals from leaving the country.

Bashir faces charges of genocide, war crimes and crimes against humanity arising out of the conflict in Darfur.