The Soviet Union said its troops in Afghanistan will observe a cease-fire today and appealed to the United States to support the truce. But U.S.-backed guerrillas vowed to keep fighting. The Soviet Foreign Ministry said that all Soviet units will lay down their arms Jan. 1 and that Moscow also is ready to stop supplying weapons to the Kabul government. It did not make clear how long the truce would last. President Mikhail S. Gorbachev had called for the Jan. 1 truce in a Dec. 7 address to the United Nations. The Kabul government also said it will observe a New Year’s truce, but guerrillas headquartered in Pakistan quickly rejected it.