(CNN) -

Israel came under attack Sunday on two fronts, according to the Israeli military.

In an incident near the Israeli-Syrian border, a 15-year-old Israeli civilian was killed and two others were injured, the Israeli military said.

"Today's event on the Syria border intentionally targeted Israelis. The explosion is the most serious attack against Israel in recent months," the Israel Defense Forces tweeted from its official account on Sunday morning.

"This is not a case of errant fire, but of an intentional attack," IDF spokesman Lt. Col. Peter Lerner said.

It was unclear whether the incident was connected to the Syrian civil war, but violence from that conflict has spilled over into northeastern Israel before.

In response, Israel fired on nine Syrian army positions, including Syrian military headquarters and launching positions, the IDF reported. The IDF reported direct hits.

On the southwestern side of the country, two rockets hit the Sha'ar Hanegev Regional Council, which borders Gaza, and an arrest was made of a "terrorist carrying a hand grenade" who the IDF believes "infiltrated Israel through the Gaza security fence," according to the IDF's blog.

In response, the IDF "targeted four terror activity sites in Gaza," the blog said.

Earlier Sunday, the Israeli military shot and killed two Palestinian men in the West Bank, medical officials said.

Mohammad Mahmmoud Ismail, 30, was killed during a raid in Ramallah. The Israeli military said it was investigating.

In addition, Ahmad Saeed Khaled, 27, was killed during a raid at a refugee camp in Nablus, a Palestinian doctor said. Khaled, a psychiatric patient, was going to morning prayers when he was shot in the legs, chest, and head, according to Dr. Ghassan Hamdan of the Palestinian medical relief society.

The Israeli military said it shot a man who approached in a threatening manner in Nablus.

"The force called on the suspect to stop and after he failed to comply, they fired warning shots in the air. The troops then fired toward the suspect, after he did not cease to advance. Initial inquiry suggests that the suspect was mentally unstable," the IDF said. An investigation is under way.

The violence comes amid an Israeli operation in response to the kidnapping of three Israeli teenagers, one of whom is reported to be a U.S. citizen.

The Palestinian medical sources say five people have died since the onset of the operation: four from Israeli military fire, and a fifth from a heart attack during a raid on his West Bank home.

An additional 463 Palestinians, including children, had been arrested or detained as of Sunday morning, according to the Palestinian Prisoners Society.