The Obama administration has significantly sharpened its rhetoric about China’s territorial claims in the South China Sea over the last week amid growing pressure from allies in the region for Washington to take a firmer line.

In public statements in recent days, senior US officials placed the blame for tensions in the region solely on China and warned that the US could move more forces to the western Pacific if Beijing were to declare a new air defence zone in the South China Sea.

Although President Barack Obama is due to visit the region in April, several Asia governments have complained privately that the administration has become distracted in the Middle East and has left the way open for China to pursue its claims with greater confidence.

“They [the administration] are definitely trying to turn up the volume about China,” said Bonnie Glaser, a China expert at the Center for Strategic and International Studies in Washington DC. “This is as close as the Obama administration has come to saying that the nine-dash line is illegal. It is quite significant because they previously danced around the issue.” The nine-dash line is a map produced by China which appears to claim that the bulk of the South China Sea is under Chinese control.

China is involved in a series of increasingly tense territorial disputes in the East China Sea with Japan and in the South China Sea with Vietnam and the Philippines. The US, along with several other governments in the region, believes that China is pushing these claims as part of a broader strategy to exert greater control over large areas of the western Pacific.

“There are growing concerns that this pattern of behaviour in the South China Sea reflects incremental effort by China to assert control over the area,” Danny Russel, assistant secretary of state for East Asia said last week at a hearing. China had “created uncertainty, insecurity and instability in the region”. Mr Russel urged China to “clarify or adjust its nine-dash line claim to bring it in accordance with the international law of the sea”.

In a separate statement, Evan Medeiros, the Asia director at the White House national security council, warned China against declaring an air defence identification zone for the South China Sea, following its announcement in December of new rules for airspace in the East China Sea.

“We have been very clear with the Chinese that we would see that [the establishment of a new air zone] as a provocative and destabilising development that would result in changes in our presence and military posture in the region,” Mr Medeiros told Kyodo, the Japanese news agency.

Speaking at a congressional hearing, Mr Russel made a series of statements that represent a hardening of the US position over the various territorial disputes. While the US claims to be neutral on the territorial disputes, he said that China was responsible for the increased tension in the region.

Mr Russel said that any claims to the seas must be based on genuine land features, rather than just rocks that can be covered at high tide. Under the UN convention on the law of the sea, a country can claim a 200km economic zone around islands. Mr Russel also endorsed the effort by the Philippines to take its territorial dispute with China to an international court, part of its efforts to find a “peaceful, non-coercive” solution.

One of the difficulties for the Obama administration is that while it bases some of its arguments on the UN convention on the law of the sea, the US Senate has refused to ratify the same treaty.