The Russian Foreign Ministry said US officials had broken into residences at Russia's consulate in San Francisco, and threatened retaliation over what it called an illegal act.Russian staff had left the consulate last month, after Washington ordered Moscow to vacate some of its diplomatic properties, part of a series of tit-for-tat actions during a thorny phase in bilateral relations.Since then, US officials had occupied administrative parts of the compound but on Monday they entered residential areas that the departing staff had locked, the ministry said in a statement late on Monday."Despite our warnings, the U.S. authorities did not listen to reason and did not give up their illegal intentions," it said. "We reserve the right to respond. The principle of reciprocity has always been and remains the cornerstone of diplomacy."Footage aired repeatedly on Russian state television showed what the broadcaster said were US officials breaking locks that had sealed off parts of the compound and entering the buildings.The "intruders" had taken over the whole premises including the consul general's residence, the ministry said."Therefore, we understand that Americans, breaking into our diplomatic buildings, have de facto agreed that their missions in Russia may be treated likewise."