Israeli Defence Minister Israel Katz said on Tuesday he had ordered the military to strike Tehran in response to what he said were missiles fired by Iran in a violation of the ceasefire announced hours earlier by U.S. President Donald Trump.
However, Iran's ISNA student news agency said reports that Iran had fired missiles at Israel after the ceasefire took effect were false.
The developments raised early doubts about the ceasefire, intended to end 12 days of war.
Katz said in a statement he had ordered the military to "continue high-intensity operations targeting regime assets and terror infrastructure in Tehran" in light of "Iran’s blatant violation of the ceasefire declared by the President of the United States."
Hours earlier, Trump had posted on Truth Social: "THE CEASEFIRE IS NOW IN EFFECT. PLEASE DO NOT VIOLATE IT!"
Israeli Prime Minister Benjamin Netanyahu had confirmed the ceasefire, saying Israel had achieved the goals it had set in launching its June 13 surprise attack on Iran, to destroy its nuclear programme and missile capabilities.
"Israel thanks President Trump and the United States for their support in defense and their participation in eliminating the Iranian nuclear threat," Netanyahu had said.
Iran's Foreign Minister Abbas Araqchi had said that Iran would halt its retaliatory strikes provided that Israel stopped attacking as of 4:00 a.m. in Tehran.
Global stock markets surged and oil prices tumbled on Tuesday after the announcement of the ceasefire, in the hope it heralded a resolution of the war just two days after the United States joined it by hitting Iranian nuclear sites with huge bunker-busting bombs.
By Maayan Lubell and Steve Holland