China's Anbang Insurance is on a shopping spree in the US hotel sector, with a nearly $13 billion offer for Starwood and the $6.5 billion purchase of 16 luxury properties from Blackstone.

Anbang, which made headlines in 2014 when it acquired the New York landmark Waldorf Astoria Hotel, is leading a consortium that has offered to buy Starwood Hotels & Resorts Worldwide, threatening Marriott International's already-agreed takeover of the company.

Starwood said on Monday it had received the $76 per share unsolicited offer, worth $12.8 billion, on March 10, topping Marriott's $63.74 per share cash-and-stock offer that Starwood had accepted last November.

Anbang is the leader in the consortium that includes China-based Primavera Capital and US private equity investor JC Flowers & Co., a person close to the matter told AFP on Monday.

As news broke about Anbang's move on Starwood, a source confirmed the Chinese insurer is buying Strategic Hotels & Resorts for $6.5 billion from The Blackstone Group, which bought the US luxury hotel group just three months ago.

Anbang and Blackstone have signed a firm agreement and the transaction could be announced in the coming days, a person with knowledge of the situation told AFP on Monday.

Anbang declined to comment on the consortium's offer for Starwood, owner of the Westin and W brands, among others. Starwood has some 1,270 properties in 100 countries.

Starwood said in a statement that its board of directors "has not changed its recommendation in support of Starwood's merger with Marriott."

The Stamford, Connecticut-base company said that it "will carefully consider the outcome of its discussions with the consortium in order to determine the course of action."

- Marriott undeterred -

Marriott, meanwhile, issued a statement saying it "reaffirmed its commitment to acquire Starwood Hotels & Resorts Worldwide, Inc. to create the world's largest hotel company."

Marriott has more than 4,400 properties in 87 countries and territories, with a portfolio of brands including The Ritz-Carlton, JW Marriott and Gaylord Hotels.

Marriott said it had been notified by Starwood of the unsolicited bid from a group led by Anbang on March 11, and had granted Starwood a waiver to consider the offer, which expires a minute before midnight on March 17.

Marriott pointed out the Anbang offer was "highly conditional and non-binding," while it had sufficient cash resources for its own bid and there was no financing contingency in the deal.

Marriott shareholders were scheduled to meet March 28 to vote on the Starwood acquisition. The Bethesda, Maryland-based company noted that if Starwood withdraws from the agreement it must pay Marriott a $400 million break-up fee.

Starwood shares jumped 7.8 percent on Monday to close at $75.93, and Marriott gained almost 3.0 percent at $70.93.

Though both Starwood and Marriott signaled Monday their preference for their tie-up, the larger offer from Anbang could sway shareholders, said Ryan Meliker of Canaccord Genuity.

"We believe it would be very difficult for the board to turn down a $76 cash offer for the hotel business, given the disparity in value vs. MAR's (Marriott's) deal," he said in a client note.

Meliker pointed out that cancellation of the tie-up would deal a tough blow to Marriott, which is counting on the merger to support its expansion, especially in China, India and Europe, regions where Starwood has a strong presence.

Anbang, which once specialized in car insurance, stormed onto the international property market in October 2014 by acquiring the famous Waldorf Astoria Hotel in Manhattan for nearly $2 billion from Hilton Worldwide Holdings.

The purchase of Strategic will bring it 16 upscale hotels and resorts in the US, including the JW Marriott Essex House in Manhattan and the Hotel Del Coronado in San Diego.

Including assumed debt in the deal, Blackstone paid about $6 billion for Strategic, a real estate investment trust, in December.

The Anbang group, which has $253 billion in assets, recently forayed into Canada. It took a controlling stake in Vancouver's Bentall Centre, and bought the HSBC building in Toronto.