Colum Eastwood has said the UK and Irish governments have been “left embarrassed” after leaving Belfast without a deal to restore devolution.
Theresa May and Leo Varadkar attended talks with Northern Ireland’s main political parties on Monday.
It was widely anticipated that the DUP and Sinn Féin were close to ending their 13-month stalemate.
But a DUP source told the BBC a deal was “not likely” on Monday and both leaders left empty handed.
Both the prime minister and taoiseach (Irish prime minister) have said they remain hopeful that a deal can be done.
But the SDLP leader said Mrs May and Mr Varadkar were left embarrassed because they travelled to Belfast to sign off on a deal that never happened.
Northern Ireland has been run by civil servants since the power-sharing executive made up of the DUP and Sinn Féin collapsed in January last year.
The then deputy first minister, Martin McGuinness, pulled Sinn Féin out of the coalition after a series of disagreements, with the final straw being the DUP’s handling of a scandal over a green energy scheme.
Previous rounds of talks between the DUP and Sinn Féin have, so far, failed to break the political deadlock.
The DUP and Sinn Féin have said that a deal is still possible.
Arlene Foster said any agreement had to be supported by everybody and sustainable.
Sinn Féin’s new President, Mary Lou McDonald, said her party is close to a deal that can be put to their grassroots.