LONDON -- Rockets have been fired from Gaza "across southern and central Israel, including Tel Aviv and Jerusalem, and the infiltration of Hamas militants," according to the U.S. embassy.

Meanwhile, the Israeli Defense Forces have declared "a state of alert for war," according to a statement.

"The IDF declares a state of alert for war. Over the past hour, the Hamas terrorist organization launched massive barrages of rockets from Gaza into Israel, and its terrorist operatives have infiltrated into Israel in a number of different locations in the south," according to the IDF.

"Civilians in southern and central Israel must remain near shelters, and in the area surrounding the Gaza Strip, inside shelters. The Chief of the General staff is currently conducting a situational assessment and approving plans for the IDF's continued activity. The Hamas terrorist organization is responsible for these attacks and will face consequences for them," the IDF continued.

In a statement issued early Saturday morning, the embassy said that it is "closely monitoring" the security situation after an unknown number of rockets were fired from Gaza into Israel in a surprise attack.

"The U.S. Embassy is aware that there have been casualties as a result of these incidents. U.S. citizens are reminded to remain vigilant and take appropriate steps to increase their security awareness as security incidents, including mortar and rocket fire, often take place without warning," the embassy said in the statement announcing the security alert.

U.S. Embassy personnel are currently sheltering in place.

This is a developing story. Please check back for updates.