Gaza Strip militants fired mortar rounds into Israel 12 hours after Saturday's start of a temporary truce Israel had agreed to extend for another four hours, the army said.
Hamas said it had not made an agreement on the truce extension, but did not explicitly reject it.
A military spokesman told AFP three mortar rounds had been fired at Israel from Gaza, causing neither casualties nor damage.
"Rockets have just been fired at Israel despite the humanitarian truce being extended" until 2100 GMT, former army spokeswoman Avital Leibovich wrote earlier on Twitter shortly after 1700 GMT, when the original truce had been due to end.
Warning sirens sounded in southern Israel shortly after 8:00 pm local time, when the original truce was due to expire.
Israeli media reports said the military did not regard the incident as a major violation of the temporary ceasefire.
Hamas said there was "no agreement" on an extension, but did not explicitly reject it.
"There is no agreement on a four-hour truce (extension)," spokesman Fawzi Barhum said in a statement.