LONDON, July 12 (Reuters) - Britain called Russia's veto of a U.N. Security Council resolution to impose sanctions on Zimbabwe "incomprehensible", and said it would try again if there is no progress in ending a crisis in the African state.

"I'm very disappointed that the U.N. Security Council should have failed to pass a strong and clear resolution on Zimbabwe," Foreign Secretary David Miliband said in a statement on Saturday.

"It'll appear incomprehensible to the people of Zimbabwe that Russia, which committed itself at the G8 to take further steps including introducing financial and other sanctions, should stand in the way of Security Council action."

"Nor will they understand the Chinese vote," Miliband said. Veto-holding China was also among five countries that opposed the U.S.-drafted text in the 15-nation council on Friday.

A spokesman for Prime Minister Gordon Brown called the veto disappointing and said it was right to seek the resolution. Britain would go back to the Council if there was no progress, he added.

"It is disappointing that the Security Council failed to stand up for the democratic rights of Zimbabweans. But it was right to push for a tough Security Council resolution, and those who stood in its way must now take responsibility for the failure of the Security Council to act," he said.

"We will continue to stand firmly for human rights and democracy, and will return to the Security Council in the absence of early progress on mediation, humanitarian access, and an end to violence."

Nine countries voted for the resolution to impose an arms embargo on Zimbabwe and financial and travel restrictions on President Robert Mugabe and 13 other officials, and authorise a United Nations special envoy for the southern African nation. One country abstained from the vote.

The Group of Eight rich nations, which includes Britain and Russia, agreed on Tuesday to impose sanctions against Zimbabwe's leadership because of violence during the widely condemned re-election of President Robert Mugabe.

Britain, Zimbabwe's former colonial power, has stood at the forefront of efforts to isolate Mugabe. London has had strained relations with Moscow over the past few years.

Despite Friday's diplomatic setback, Miliband insisted the government would keep up pressure on Mugabe.

Zimbabwe's opposition Movement for Democratic Change leader Morgan Tsvangirai withdrew from a June 27 presidential run-off poll, citing attacks on his supporters by pro-Mugabe militia.

The MDC and Western powers have branded Mugabe's landslide re-election a sham.

"We will continue to advocate intensified EU measures against Mugabe and his ruling clique. The U.N. still has a key role to play in supporting African efforts to bring an end to this crisis, and we will continue to press for the appointment of a U.N. envoy," Miliband said.

"A solution must be found that reflects the will of the Zimbabwean people, whose will continues to be so brutally denied."

(Reporting by John Joseph and Peter Graff; Editing by Catherine Evans)

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