The United States, Britain, France, Germany, Spain, Italy, Canada and Malta issued the joint statement saying they were "deeply concerned" about the situation in the North African state.
"We urge all parties to cease all military operations and to refrain from taking any steps which increase the polarization and divisions in the country," they said.
The eight countries have issued joint statements in the past urging an end to the violence, as conditions there have spiraled out of control.
The parliament, which is recognized by the international community, immediately rejected the Supreme Court ruling as made "under the threat of arms."
"We are studying carefully the decision of the Supreme Court, its context and consequences," the statement by the Western powers said.
Tripoli, where the Supreme Court has its seat, is controled by a coalition of Islamist militias, Fajr Libya (Libya Dawn), which have created a parallel "parliament" and "government."
The parliament, which is dominated by opponents of the Islamist militias, now meets in Tobruk for security reasons rather than in Benghazi.
