WASHINGTON (Reuters) - U.S. Treasury Secretary Steven Mnuchin said on Tuesday that if China doesn t follow the United Nations sanctions approved on North Korea, he will seek new financial sanctions on Beijing to cut off access to the U.S. financial system.  Mnuchin told a conference broadcast on CNBC that China agreed to  historic  sanctions on North Korea on Monday in a UN Security Council vote.  If China doesn t follow these sanctions, we will put additional sanctions on them and prevent them from accessing the U.S. and international dollar system, and that s quite meaningful,  Mnuchin said.    