Israel's air campaign on Iran's capital appeared to broaden on the fourth day of an intensifying conflict as U.S. President Donald Trump posted an ominous message warning residents of the city to evacuate.

The risk of conflict is growing as Iran signals strikes on Israeli nuclear facilities despite ongoing nuclear talks.

Donald Trump said late on Monday his early departure from the Group of Seven nations summit has "nothing to do with" working on a ceasefire between Israel and Iran, denying comments by French President Emmanuel Macron who said the U.

Israel unleashed airstrikes across Iran for a third day and threatened even greater force as some Iranian missiles evaded Israeli air defenses to strike buildings in the heart of the country.

President Donald Trump abruptly announced that he would leave the G7 summit a day early and return to Washington shortly after posting an ominous social media message that appeared to suggest more carnage in Iran.

Israeli forces stepped up bombing of Iranian cities, while Iran pierced Israeli air defences with a successful volleys of retaliatory missile strikes.

Iran and Israel continued to exchange attacks into Monday amid the latest round of conflict between the two nations.