Chinalco and Alcoa - through a Singapore-based investment company - snapped up a combined 12% stake of Rio for $US14 billion ($A15.5 billion) on Friday, equating to a 9% stake in its global operations and sparking rumours of a possible rival bid to BHP's proposal.

However, analysts believe Chinalco and Alcoa were primarily on the hunt for Rio's aluminium assets - including its subsidiary Rio Tinto Alcan's Gladstone and Yarwun operations - as neither had significant interests in Rio's other commodities.

Fat Prophets analyst Gavin Wendt told MiningNews.net that he thought the stake had not been bought to potentially acquire Rio but instead was a move to take a seat at the bargaining table.

"I don't think the Chinese have any intention [of a possible takeover bid], I think they are smart enough to realise that any takeover wouldn't be allowed by Australian authorities," he said.

Wendt said the investment had thrown a spanner in the works of BHP's move on Rio prior to the February 6 deadline imposed by the UK Takeover Panel's "shut up or put up" clause.

"It's a brilliant move really because it stuffs BHP up a couple of days before its deadline and I think it will be very hard for BHP by Wednesday to come up with an answer," he said.

"I wouldn't imagine BHP would be able to go away in the space of a couple of days and come back with a firm bid - and it is going to have to be a higher one now as a result of what the Chinese and Alcoa have done.

"Even if a bid does go ahead, some day down the track BHP is going to have to walk away for six months minimum as specified by the takeover code in the UK and they are going to have to come back with a higher offer."

Wendt said as a result of having to pay more for Rio, BHP may have to look at selling off assets, including Rio's aluminium-alumina operations.

"If BHP does mount a successful bid for Rio, it positions the Chinese and Alcoa to pick up the spoils of that merger, which is the aluminium-alumina assets of Rio, the jewel in the crown being the Alcan assets which Alcoa were unsuccessful in bidding for last year," he said.

"So there is really no downside for them as far as I can see."

Meantime, according to a report in the Australian Financial Review, Chinalco president Xiao Yaqing arrived in Sydney yesterday to meet with the Federal Government to discuss Chinese government-backed investment in Australia's mining sector.

The transaction comes after a recent report in China Daily which said Chinalco had been looking to build an alumina base in Australia.

Last week, speculation was mounting that Rio had opened its books to BHP on the proviso that BHP increases its proposed bid from three of its shares for each Rio security, to three and a half of its shares for one Rio share.

Shares in Rio have gained $3.67 to $130.98 in morning trade, while BHP has gained $1.21 to $39.76.