The United States and Cuba failed Friday to nail down an agreement on reopening their embassies and more discussions to restore long-frozen diplomatic ties will be needed, both sides said.

The United States and Cuba failed Friday to nail down an agreement on reopening their embassies and more discussions to restore long-frozen diplomatic ties will be needed, both sides said.

"Both delegations agreed to continue our exchanges on issues related to the functioning of diplomatic missions so we will continue those conversations in the next few weeks," the head of the Cuban delegation, Josefina Vidal, told reporters after two days of talks.

She refused to go into detail about exactly where the differences lay between the Cold War foes, who are seeking to overcome half a century of enmity and restore normal ties.

The fourth meeting between the two sides ran into an unscheduled second day on Friday at the State Department in Washington.

"This has not been an easy task given our complicated history," top US diplomat for Latin America Roberta Jacobson told reporters in a separate press conference.

"This round of talks was highly productive. We will persist, inspired by the conviction that engagement and not isolation are the keys to moving forward."

The two sides had "made significant progress in the past five months" and were "much closer" to re-establishing relations and reopening their respective embassies.

She suggested that another full round of talks might not be needed, with the discussions being continued by diplomats from the two countries' interests sections in Havana and Washington.

"I think that we made a great deal of progress this time and I don't know that we will need another round," Jacobson said.

In a ground-breaking move, President Barack Obama and Cuban President Raul Castro agreed in December to seek a normalization of relations frozen for five decades.

"But we're still talking about various aspects of the functioning of an embassy," Jacobson said Friday, refusing to be more specific about the sticking points.

The United States has so far announced it plans to remove Cuba from a blacklist of state sponsors of terror, which should happen around May 29 after a 45-day review period by Congress.

Havana has also found a bank willing to handle its financial affairs on US soil, something which had been a major hurdle.