Russia and China have blocked a US move to add a Russian bank to a UN sanctions blacklist along with a North Korean official and two entities, diplomats said.
The United States last week asked a UN sanctions committee to slap an assets freeze on Agrosoyuz Commercial Bank for allegedly helping North Korea evade UN-imposed restrictions on financial transactions.
The request also targeted Ri Jong Won, the deputy representative of North Korea's Foreign Trade Bank and two North Korean front companies.
In a response to the council on Thursday, Russia raised doubts about the allegations while China told the council that it objected to the proposed sanctions designations put forward by the United States.
"We would like to underline that designation requests submitted to the committee should be adequately substantiated by sufficient information," Russia's UN mission said in a message to the council.
The request followed a US Treasury Department decision to impose unilateral sanctions on the Russian bank, the North Korean official and the two entities.
The two companies were named as the Dandong Zhongsheng Industry & Trade Company and the Korea Ungum Corporation.
Russia and China have called on the Security Council to consider easing sanctions to reward North Korea for opening up dialogue with the US and halting missile tests.
But the United States has called for maintaining "maximum pressure" from sanctions until North Korea has fully dismantled its nuclear and ballistic missile programmes.
Earlier Thursday, the North Korean foreign ministry denounced the US for responding to its overtures by "inciting international sanctions and pressure".
The council last year adopted three rafts of sanctions targeting North Korea's economy through export and import bans, as well as restrictions on banking.
It was the second time in three weeks that Russia and China have objected to a US request to tighten sanctions on North Korea.
On July 19, the two countries put a six-month hold on a US request to halt all deliveries of refined oil products to North Korea.
The United States had made the request to the sanctions committee after finding that North Korea had exceeded the cap on fuel imports though illegal ship-to-ship transfers at sea.
UN sanctions resolutions place a ceiling for North Korea of 500,000 barrels of refined oil products per year and four million barrels of crude.
A cut-off of oil and fuel would have to be enforced primarily by China, which supplies most of North Korea's energy needs, but also by Russia, which delivers some oil to Pyongyang.
Meanwhile, South Korean customs officials said three local firms were caught importing coal and iron from the North via Russia.
More than 35,000 tonnes of North Korean coal and iron were imported into the South via Russia between April and October last year, the Korea Customs Service said.
The coal shipments were first sent to Russia, where their details were disguised using forged "country of origin" documents, and then reloaded on ships bound for the South, the customs office said after a 10-month investigation by the authorities.
"Korea Customs Service has confirmed seven criminal offences and it will report three persons and three companies to prosecution authorities with a request to indict them," it said.
News of the apparent breach comes after a UN report last week accused the North of flouting sanctions by continuing to export coal, iron and other commodities as well as carrying out illegal ship-to-ship transfers of oil products at sea.