HAVANA (REUTERS) - The United States and Cuba launched talks on Thursday on restoring diplomatic relations after a contentious session on immigration accentuated the difficulties in overcoming half a century of hostilities.

The highest-level US delegation in 35 years will conclude two-day talks in Havana on Thursday, with both sides cautioning an immediate breakthrough was unlikely.

Senior US officials say they hope Cuba will agree to reopen embassies and appoint ambassadors in each other's capitals in coming months.

The United States also wants travel curbs on its diplomats lifted and unimpeded shipments to its mission in Havana.

During talks on Wednesday, the Americans vowed to continue granting safe haven to Cubans with special protections denied to other nationalities.

Cuba complained that the US law promotes dangerous illegal immigration and protested against a separate US programme that encourages Cuban doctors to defect, calling it a "reprehensible brain drain practice."

As her deputy sparred with the Cuban officials over immigration policy, the lead US negotiator in the diplomatic talks, Ms Roberta Jacobson, arrived in Havana aboard a commercial charter from Miami.

She became the first US assistant secretary of state to travel to the communist-led island in 38 years and the highest-ranking visitor in 35 years.

Her Cuban counterpart will be Ms Josefina Vidal, director of the foreign ministry's US affairs, who also participated in the immigration talks.

The meetings are the first since US President Barack Obama and Cuban President Raul Castro announced on Dec 17 they would work to restore diplomatic ties snapped by Washington in 1961.

Despite resistance from some in Congress, Mr Obama has set the United States on a path toward removing economic sanctions and a 53-year-old trade embargo against Cuba.

US Secretary of State John Kerry said on Wednesday he looked forward to formally opening a US embassy in Cuba.

Mr Kerry also said he was prepared, when the time was right, to meet his Cuban counterpart Bruno Rodriguez, with whom he has only talked by telephone.

"And when it is timely, when it is appropriate, I look forward to traveling to Cuba in order to formally open an embassy and begin to move forward," Mr Kerry told reporters in Washington.

In his annual State of the Union speech on Tuesday, Mr Obama urged Congress to start work on ending the embargo but critics say Obama first needs to win concessions on Cuban political prisoners and democratic rights, the claims of US citizens whose property was nationalized after Cuba's 1959 revolution, and US fugitives who have received asylum in Cuba.