Israel's military said it killed 15 Hezbollah members in south Lebanon on Thursday and bombed Beirut after its forces suffered their deadliest day on the Lebanese front in a year of clashes with Hezbollah.

Israel, which has been fighting with Hamas in Gaza for almost a year, sent its troops into southern Lebanon after two weeks of intense airstrikes, in an escalating conflict that risks drawing in the United States and Iran.

Lebanon's health ministry said the death toll in the Beirut strike had climbed to nine and the Islamic Health Authority, a Hezbollah-linked civil defense group, said seven of its staff including two medics had been killed.

Reuters witnesses reported hearing a massive blast, which a security source said had targeted a building in the district of Bachoura a few hundred meters from parliament, the closest an Israeli strike has come to the central downtown district of Beirut.