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Samsung's tablets infringe patents owned by Apple, a US judge has ruled.

However, she warned that Apple needs to prove the validity of those patents if it is to win an injunction preventing the sale of Samsung's Galaxy Tab.

The decision is the latest in a dispute which spans 20 cases in 10 countries, some of which have led to Samsung products being taken off the shelves.

In a separate case in the Netherlands, a court rejected a similar bid by Samsung to ban Apple products.

Samsung had argued Apple should not be allowed to sell devices that use 3G mobile technology patented by Samsung because it did not have licenses to use the technology.

Apple claims that the Galaxy range "slavishly" copies its massively-selling iPad and iPhone.

The Californian company has sought to ban the sale of the Galaxy S 4G smart phone and Galaxy Tab 10.1 tablet in the US.

A similar ruling in an Australian court led to a temporary sales ban on the tablet in that country.

US mobile networks have opposed Apple's request, saying it could affect sales of the products in the run up to Christmas.

California District Judge Lucy Koh ruled that while patents have been infringed, Apple must now show that its claim on those inventions is valid.

The disputed patents include three covering design - including the exterior look of the device - and a 'list scrolling patent' which relates to how users view documents.

Samsung argued that Apple's claims are invalid due to previously registered patents relating to design elements and functionality.

The company's attorney, Kathleen Sullivan, said her client "clearly raised substantial questions" and that the injunction bid should therefore be rejected.

But Apple argue that its own product's design is far superior to those which came before the iPad and iPhone, and so therefore its patents should not be invalidated.

At one point in the hearing the judge held both Samsung and Apple products up on the air and challenged the defence to whether they could identify which device was which.

Samsung attorney Ms Sullivan, who was roughly 10 feet away, responded: "Not at this distance your honour."

Another lawyer for Samsung correctly distinguished the two.

A spokesman for the South Korean company described Apple's claims as "groundless".

Apple spokeswoman Kristen Huget said: "It's no coincidence that Samsung's latest products look a lot like the iPhone and iPad.

"This kind of blatant copying is wrong, and we need to protect Apple's intellectual property when companies steal our ideas."

Both companies have been locked in patent disputes since April, with each accusing the other of infringing various patents.

In The Hague, a Dutch court refused a Samsung bid to block Apple products on the basis that the US company did not have the right to 3G licenses.

The ruling said that 3G technology has been accepted as an industry standard, and therefore Samsung was obliged to offer Apple licenses according to "fair, reasonable and nondiscriminatory" - or Frand - terms.