Damascus and Detlev Mehlis, the UN chief investigator, have been discussing the venue and legal framework for questioning six senior Syrian security officials.
A unanimous Security Council resolution last month demanded that Syria should co-operate fully with the investigation into the killing of al-Hariri, who died in a truck bombing in Beirut on February 14.
Farouq al-Shara, the Syrian Foreign Minister, in a letter on Monday "asked the help of the president of the Security Council and the UN secretary-general in agreeing a cooperation protocol" with Syria, a Foreign Ministry official said on Tuesday.
But Stephane Dujarric, chief UN spokesman, said the secretary-general would not be getting involved and that it was up to Mehlis how he conducted the interviews.
The Security Council would also not intervene, US Ambassador John Bolton said when asked about the Syrian letter.
"We support Mehlis's investigative efforts and we are not going to second-guess his techniques or his tactics," he said. "We are going to say clearly to the Syrians that they need to stop delaying and obstructing Mehlis's work."
President al-Assad said Syria
would co-operate with the UN
Mehlis returned to Lebanon on Tuesday to prepare to wrap up the inquiry. He has until 15 December to submit his final report, but diplomats say he might go to the Security Council earlier if he gives up on Syrian co-operation.
Lebanese sources said on Monday that Mehlis had rejected a proposal to use UN offices in the Golan Heights to question the Syrian officials.
Syria has resisted letting the interviews take place in Beirut.
Mehlis, a German prosecutor who leads the UN inquiry, had demanded to be allowed to question the six military and intelligence officials at his headquarters in Lebanon.