Lawyers representing the WikiLeaks founder, Julian Assange, say that they have been surveilled by members of the security services and have accused the US state department of behaving "inappropriately" by failing to respect attorney-client protocol.

Jennifer Robinson and Mark Stephens of the law firm Finers Stephens Innocent told the Guardian they had been watched by people parked outside their houses for the past week.

"I've noticed people consistently sitting outside my house in the same cars with newspapers," said Robinson. "I probably noticed certain things a week ago, but mostly it's been the last three or four days."

Stephens said he, too, had had his home watched. Asked who he thought was monitoring him, he said: "The security services."

Robinson said the legal team was also experiencing "other forms of pressure" from Washington.

She pointed to a letter from a state department legal adviser – addressed to both Assange and her – which appeared to bracket together client and lawyer as if to suggest that WikiLeaks and its lawyers were one and the same.

The letter, which was released to the press, begins: "Dear Ms Robinson and Mr Assange. I am writing in response to your 26 November 2010 letter to US Ambassador Louis B Susman regarding your intention to again publish on your WikiLeaks site what you claim to be classified US government documents."

Robinson said: "By eliding client and lawyer, that was a very inappropriate attempt to implicate me. That is really inappropriate to come from the state department of all places; they understand very well the rules on attorney-client protocol."

She said that although they had requested a public retraction from the state department, no answer had been received.

"It's quite a serious situation," she said, adding that, according to the UN's Basic Principles on the Role of Lawyers, governments should ensure that lawyers "are able to perform all of their professional functions without intimidation, hindrance, harassment or improper interference" and that "lawyers shall not be identified with their clients or their clients' causes as a result of discharging their functions".

A spokeswoman for the Home Office declined to comment on the lawyers' surveillance claims.

Assange, who is staying in Britain, has come under growing pressure from politicians in the US and around the world after his site started publishing excerpts from a cache of 250,000 secret American diplomatic cables last week.

It emerged on Saturday that Australian police are investigating whether Assange, an Australian citizen, has broken any of the country's laws and is liable to prosecution there.

The foreign minister, Kevin Rudd, said: "The Australian government unequivocally condemns the action by any of those responsible for the unauthorised release of classified and confidential information and diplomatic communications between states."

Swedish prosecutors have sent an international arrest warrant to the Metropolitan police, seeking the extradition of Assange for questioning on allegations – which he strongly denies – of rape, sexual molestation and unlawful coercion.

Stephens today denounced the extradition warrant as a "political stunt" and said Assange would fight deportation to Sweden on the grounds that it could lead to him being handed over to the US, where senior politicians have called for him to be executed.

The former vice-presidential candidate Sarah Palin has described him as "an anti-American operative with blood on his hands" and called for him to be hunted down like a Taliban leader, while another senior Republican, Mike Huckabee, has said that "anything less than execution is too kind a penalty" for his actions.

Stephens said that Sweden's chief prosecutor had told Assange in September that there was no case for him to answer, following complaints against him by two women, but the investigation was revived following the intervention of a Swedish politician.

He said that Swedish prosecutors knew where Assange was and urged them to call him to discuss the case.

Stephens told BBC1's Andrew Marr Show: "It is quite bizarre, because the chief prosecutor in Sweden dropped the entire case against him, saying there was absolutely nothing for him to find back in September, and then a few weeks later on – after the intervention of a Swedish politician – a new prosecutor, not in Stockholm where Julian and these women had been, but in Gothenburg, began a new case which has resulted in these warrants and the Interpol Red Notice being put out."

He added: "It does seem to be a political stunt."