The wording of the article changed from earlier this morning: (as quoted at Israel Matzav):

The IDF on Thursday afternoon said that the army ordered an artillery shelling of the UNRWA building in Gaza only after a Givati Brigade force came under anti-tank fire from within the building. Channel 10 reported that artillery pieces situated outside the Strip pounded the building.

Then the update version at jpost.com:

Gunshots and an anti-tank missile were fired at IDF troops near the UN compound that was attacked by the IDF on Thursday, senior defense officials told The Jerusalem Post.

According to the officials, the IDF responded by firing artillery shells at the location of the gunmen, causing damage to the UN installations. At least three people were wounded in the attack and the building was set on fire.

Which is it?