U.S. Secretary of State John Kerry said on Monday that he had never spoken of Israel as a country linked to apartheid, an episode in the stalled Israeli-Palestinian peace talks that was sparking an outcry both at home and abroad.

"I will not allow my commitment to Israel to be questioned by anyone, particularly for partisan, political purposes, so I want to be crystal clear about what I believe and what I don't believe," Kerry said in a written statement.

"Israel is a vibrant democracy and I do not believe, nor have I ever stated, publicly or privately, that Israel is an apartheid state or that it intends to become one," he added. "Anyone who knows anything about me knows that without a shred of doubt."

He cited his strenuous efforts to broker a peace deal between the Israelis and Palestinians for over one year as his concrete support for Israel.

The top American envoy was facing a call for resignation and sharp rebuke over his alleged remarks that Israel risks becoming "an apartheid state" if there is no two-state solution to its conflict with the Palestinians.

"Secretary Kerry has proven himself unsuitable for the position he holds ... John Kerry should offer President (Barack) Obama his resignation," said Senator Ted Cruz on Monday.

"Kerry, shame on you. There are some words you cannot use," Israeli Transport Minister Israel Katz wrote on his Facebook page.

Negotiations Kerry initiated in late July came to a deadlock recently as the two rivals refused to budge their positions. He has since then been focusing on extending the talks beyond the original deadline of April 29.