An agreement on hostage exchange and humanitarian pause in Gaza is still possible but "very hard' issues still need to be resolved, U.S. Secretary of State Antony Blinken said in Tirana on Thursday.

Blinken was speaking at a news conference during a visit to Albania days after talks involving the U.S., Egypt, Israel and Qatar on a deal that would see a pause in fighting in Gaza ended without a breakthrough on Tuesday.

"So we're very focused on it and I believe it's possible," he told a joint news conference with Albanian Prime Minister Edi Rama, as negotiations between mediators continue in Cairo.

Mediators are racing to secure a pause to the fighting before Israel proceeds with a full-scale ground incursion into the Gaza Strip's far-southern city of Rafah, where more than 1.4 million Palestinians are trapped.

The potential for mass civilian casualties has triggered urgent appeals, even from close allies, for Israel to hold off sending troops into the last major population center they have yet to enter in the four-month war.

Key ally the United States has said it will not back any ground operation in Rafah without a "credible plan" for protecting civilians.

Rafah is the main entry point for desperately needed relief supplies and U.N. agencies have warned of a humanitarian disaster if an assault goes ahead.

U.N. humanitarian chief Martin Griffiths said any military operation "could lead to a slaughter."