On Sunday Zimbabwe announced a delay in a partial recount of the March 29 parliamentary vote, extending a deadlock that the opposition says is a ploy by Mugabe to prepare for a presidential run-off.
AP reported on Tuesday that US intelligence agencies were tracking the An Yue Jiang.
The news agency quoted Kurtis Cooper, a US state department spokesman, as saying that the arms "could be used against individuals who are merely trying to freely express their political will".
"We call on the Zimbabwe government to immediately cease the perpetration of brutal acts against its citizens and strongly urge the regional leadership to enhance its intervention for an expeditious solution to the post-election crisis."
Jendayi Frazer, the department's Africa expert, is expected to visit the region to underscore US concerns about the arms shipment, and to persuade Zimbabwe's neighbours to pressure the government to publish the election results.
The government has also refused to publish the results of the presidential election, which the opposition claims to have won.
The ruling Zanu-PF party is fighting to stay in power, and there are reports of increasing violence against the opposition.
Zanu-PF officials say Zimbabwe has a sovereign right to defend itself and buy weapons from "any legitimate source worldwide".
"I don't understand all this hullabaloo about a lone ship," Patrick Chinamasa, the country's justice minister, said in Harare.
"We don't need clearance from anyone."
The An Yue Jiang, packed with ammunition, rockets and mortar bombs, had to abandon plans to offload in South Africa on Friday after dock workers won a court order barring it from transporting the cargo overland to Zimbabwe.
The 300,000-strong South African Transport and Allied Workers Union refused to unload the weapons in Durban, citing concerns that the government of Robert Mugabe might use them to break Zimbabwe's political stalemate.
|"We don't need clearance from anyone"|
Patrick Chinamasa, Zimbabwe Justice Minister
After Mozambique's refusal for entry into its waters, the An Yue Jiang was reportedly headed to Angola, whose president is a long-time ally of Mugabe.
Filomeno Mendonca, director of the Institute of Angolan Ports, said preventive measures have been taken although the An Yue Jiang has not requested to dock.
"We have warned our ports that this ship does not have authorisation to enter in Angola and therefore will not be assisted in Angola," Mendonca told a local radio station.
"I'm waiting for a message from South Africa, giving me the name of the ship and the situation so I can control the situation better."
Zambia called on regional governments to bar the ship from entering their waters, suggesting that the Chinese could play "a very useful role" without offering Zimbabwe firearms.
"I hope this will be the case with all the countries because we don't want a situation which will escalate the [tension] in Zimbabwe more than what it is," Levy Mwanawasa, the Zambian president, said.