In a case which has badly soured diplomatic relations between the two countries, the two marines are accused of firing shots which killed the Indians while they were fishing off the southern Indian state.
The marines were on board an Italian oil tanker, providing protection as part of an international anti-piracy mission.
India detained the two marines days after the fatal incident and a court case is pending.
Italy disputes India's jurisdiction over the case and, having grown frustrated with the slow pace of the Indian legal system, decided in June to make a unilateral request for the Hamburg-based International Tribunal for the Law of the Sea to make a ruling.
In a provisional order on Monday, the international court said: "Italy and India shall both suspend all court proceedings and shall refrain from initiating new ones which might aggravate or extend the dispute submitted."
That in effect is a decision in favour of Italy, which had asked that India halt its judicial actions while awaiting the court's final ruling.
But the UN court did not accede to Rome's second request for both marines to be freed immediately pending a final ruling.
Francesco Azzarello, who represented the Italian government at the trial, welcomed the tribunal's recognition that it was the competent body to make a judgement on the episode.
But he added: "We are nevertheless disappointed that the tribunal did not make any decision on the situation of (the two marines) Latorre and Girone on the grounds that this was a matter for the arbitration panel being constituted.
"For this reason, Italy is considering making a new application relative to the status of the marines as soon as it is established."
In a statement released by the Foreign Ministry in Rome, Azzarello added: "We are certain that India will apply without delay the measures decided today."
Sore point in Italy
One of the servicemen, Massimiliano Latorre, was last year allowed to temporarily return to Italy for medical treatment after he suffered a blood blockage in his brain. His permission to remain in Italy for recuperation was extended by six months in July.
The other marine, Salvatore Girone, has been living at Italy's embassy in New Delhi.
Earlier in the hearing, Azzarello argued that the marines had not committed any crime.
He said the oil tanker, the MV Enrica Lexie, was in international waters at the time of the incident and accused India of an "unlawful exercise of jurisdiction over the incident, over the vessel and over the marines" whom it had arrested.
India argues that the case is not a maritime dispute but "about a double murder at sea", in which one fisherman was shot in the head and the other in the stomach.
The detention of the marines and the long delay in the case coming to trial is a sore subject in Italy with Prime Minister Matteo Renzi's government frequently being flayed by the opposition over its failure to bring both men home.
Right-wing politicians on Monday branded the Hamburg ruling as another defeat for Renzi. "It is deeply disappointing. The tribunal, in effect, decided to decide nothing," said Maurizio Gasparri, a senator with Silvio Berlusconi's Forza Italia party.