Rockets Intercepted Over Jerusalem and Tel Aviv, Israel Amasses Troops at Gaza Strip

— -- Bomb shelters in Jerusalem have been opened after a rocket launched from the Hamas-controlled Gaza Strip was intercepted Tuesday evening, authorities said.

That rocket launch came after militants fired two rockets at Tel Aviv, Israel's commercial capital. Both projectiles were intercepted by the "Iron Dome" rocket defense system, authorities said.

The second interception was shown on live TV, as a winding plume of smoke followed the interceptor into the black skies.

All told, the West Bank militants fired about 160 rockets at Israel, while Israel said it attacked more than 150 sites across Gaza.

Palestinian medics reported at least 25 dead, including six killed in an airstrike that flattened an apartment building in southern Gaza and set off widespread panic.

The Israeli military said one rocket exploded in the northern Israeli city of Hadera, about 60 miles away from Gaza, in the farthest a rocket from Gaza has reached so far, but they added that there were no reported injuries.

This Is What It's Like Inside Gaza During Missile Strikes

In response, Israel built up forces along the border, the government authorized the army to activate up to 40,000 reservists. The army said that about 1,000 soldiers were immediately activated, in addition to an earlier group of 1,500 reservists called into action.

Israel and Hamas are bitter enemies that have engaged in numerous rounds of fighting over the years. But until recently, they had been observing a truce that ended the previous hostilities in 2012.

Local reports claimed that Israeli Defense Forces killed five men that they identified as Hamas terrorists commandos after they allegedly tried to infiltrate an IDF army base by sea.

The men reportedly exchanged fire with IDF members on Zikim beach.

Tensions have been rising since Palestinian militants kidnapped three Israeli teenagers in the West Bank on June 12.

Accusing Hamas of being behind the abductions, Israel launched a crackdown on the group's members in the West Bank and arrested hundreds of people. Hamas, which controls Gaza, responded by stepping up rocket fire.

The situation deteriorated last week after the bodies of the Israeli youths were found, and a Palestinian teenager in Jerusalem was abducted and burned to death in what Palestinians believe was a revenge attack.

Sirens have gone off in Jerusalem as rockets that appear to be from the Hamas-controlled West Bank continue to target Israel.

Two sets of air raid sirens have gone off in Tel Aviv this afternoon and Jerusalem is the latest area to be targeted.

The latest round of alarms comes after Israel launched its largest offensive in the Gaza Strip in nearly two years by carrying out a blistering aerial assault.

The offensive set off the heaviest fighting between Israel and the Islamic militant group Hamas since an eight-day battle in November 2012.

Residents have posted videos of the siren sounding to social media accounts.

One video, posted by Instagram user luizszutman, shows a man standing in a stairwell holding his ears as the siren blares in Tel Aviv.

Another shows a group running through the streets as the siren goes off.

The sirens sounded as the Israel Conference on Peace was being held at the InterContinental Hotel in Tel Aviv.

"The sirens went off outside and we couldn't hear it but the security told us to gather in the main hall. As the video can tell, it wasn't so scary but the irony is clear," Instagram user Nirtzur, who shot the video of crowds gathered outside the hotel, told ABC News.

The first reported rocket launch this afternoon came hours after the Israeli military struck more than 100 sites in the Hamas-ruled Gaza Strip.

The Associated Press contributed to this report.