China agreed Tuesday to extend energy-rich Turkmenistan an extra $4.1 billion in credit, a loan that officials said will be paid off with future supplies of natural gas. State energy company Turkmengaz said the funds will be used to boost Turkmenistan’s export potential and develop the vast South Yolotan field in the east of the country. The loan puts Turkmenistan in further debt to China, which in 2009 issued the Central Asian nation soft credit worth $4 billion.