British prime minister David Cameron has called on U.S. drugmaker Pfizer to give stronger guarantees it will keep jobs and investment in the UK if it is to secure his government's blessing for a takeover of AstraZeneca.

By proposing the biggest ever foreign takeover of a British firm, Pfizer has sparked fierce debate on whether the government should let outsiders buy a pharmaceuticals group seen as a national champion in a strategically vital industry.

The government initially welcomed the bid as a vote of confidence in a tax system it has designed to attract investors. But after intense pressure in parliament, Cameron said on Wednesday that Britain had to be hard-headed and indicated he even supported possible intervention to hold up the purchase.

"The commitments that have been made so far are encouraging," Cameron said when asked about the $106-billion takeover offer during his weekly question session in parliament.

"But let me absolutely clear, I'm not satisfied. I want more. But the way to get more is to engage," he added.

"We want the investment, the jobs and the research that comes with that competitive tax system."

Though the government has only limited legal power to block any merger, Pfizer CEO Ian Read has already given a five-year commitment to complete AstraZeneca's new research centre in Cambridge, retain a factory in Macclesfield and put a fifth of its research staff in Britain if the deal goes ahead.

Cameron did not directly say whether he wanted to apply a public interest test to the Pfizer proposal but he said he agreed with business secretary Vince Cable who said earlier the government could apply the test, asking regulators to judge whether the deal was in the national interest.

Imposing such a test could further complicate Pfizer's bid - already twice rejected by AstraZeneca - to create the world's largest pharmaceuticals business and save Pfizer billions of dollars in taxes by shifting its domicile to Britain.

Read and AstraZeneca CEO Pascal Soriot will appear before two parliamentary committees next week to explain the merger.