Smoke billows over Gaza City after Israel's military targeted Hamas posts in Gaza in retaliation to mortar rounds fired from the Palestinian enclave. (Mahmud Hams/AFP/Getty Images)

Israel said it bombed six targets in Gaza on Thursday in response to a volley of mortar fire from Palestinian militants, as tensions escalate in the coastal enclave.

About a dozen mortar rounds were fired Thursday afternoon at Israeli army forces involved in construction work along the fence that separates Israel from Gaza, according to Lt. Col. Jonathan Conricus, a spokesman for the Israeli military. There were no casualties, he said.

Conricus said the mortars probably were fired in retaliation for Israel's destruction a month ago of an attack tunnel that snaked from Gaza into Israel that the Islamic Jihad militant group was building. Seven Palestinians were killed in the initial attack, and Israel later said it had recovered the bodies of five more militants who were reported missing.

After the mortar fire, Israeli jets targeted four military sites belonging to Hamas and Islamic Jihad, Conricus said. Later on Thursday evening, the army said it also had struck two "terror infrastructure targets." At least two Palestinians were injured, according to Gaza's Health Ministry.  

Israel and Hamas, the militant Islamist group that controls the Gaza Strip, have fought three wars since 2008. But the tunnel incident also has increased friction between Israel and Islamic Jihad, the second-largest militant group in the strip, which had threatened revenge for the deaths of its fighters. 

"The Israel Defense Forces sees this as a severe event," Conricus said of the mortar fire. "We know exactly who conducted this attack, down to their names, but we hold Hamas responsible for any hostility from Gaza."

Daud Shehab, the leader of Islamic Jihad in Gaza, said the "Zionist aggression" was an attempt to distract from the killing of a Palestinian man in the West Bank by settlers earlier in the day. 

"The Palestinian resistance has the full right to defend and respond to this dangerous escalation. The occupation bears responsibility," he said. 

A few days after the tunnel bombing, Israel's military said it was holding onto the bodies of the five militants, which could be used as a bartering tool to secure the release of the bodies of two Israeli soldiers killed in Gaza during the 2014 summer war and three Israeli civilians who disappeared in the enclave between 2014 and 2016.

Israel has been developing technology to detect and destroy tunnels since its 2014 war with Hamas, when the group's members used underground warrens to launch attacks and defend themselves from airstrikes. A total of 67 Israeli soldiers and six Israeli civilians were killed in the conflict, while 2,251 Palestinians died, according to the United Nations.

Israel also is building an underground barrier to prevent tunnels from crossing into Israel.

Balousha reported from Gaza. Loveday Morris in Jerusalem contributed to this report.