Israeli air force jets have struck Hamas positions in the Gaza Strip after rockets were fired on Friday from the territory towards Israel, the occupation forces said.

The exchange came after Hamas warned that Israel's planned annexation of parts of the occupied West Bank amounted to a "declaration of war".

Israel jets struck ammunition and rocket "factories" in the southern Gaza Strip, the Israeli forces said.

Security sources in Gaza confirmed to AFP there were strikes in the area of Khan Younis, at the southern tip of the Palestinian territory of 2 million inhabitants.

Two rockets were fired on Friday from the Hamas-controlled Gaza Strip towards Israeli territory, the Israli forces said earlier.

"Two rockets were fired from the Gaza Strip towards Israeli territory," the Israeli military said in a statement, after saying "sirens sounded in the area surrounding the Gaza Strip".

Medics said the sirens sounded in the Israeli district of Sderot.

It was the first reported rocket fire from the Gaza Strip since early May.

Israel's proposal to annex its settlements in the West Bank and the Jordan Valley form part of a broader US peace plan released in January.

The proposals foresee the ultimate creation of a Palestinian state on the remaining West Bank territory and the Gaza Strip.

But the plan falls far short of Palestinian aspirations, with a state on reduced territory and without East Jerusalem as its capital.

Israel has waged three aggressions on Gaza in recent years, with the latest one in 2014 killing 2,251 Palestinians.

There are no official relations between the two sides. The Palestinian Authority, a separate administration based in the West Bank, cut its cooperation with Israel last month.