International lenders and markets are growing increasingly concerned about "hidden debt" in Zambia, a situation similar to the one that plunged neighbouring Mozambique into a deep financial crisis.For several weeks financiers have demanded answers about the level of the country's true external borrowing - but officials have rubbished any suggestion of concealed debts.Zambia, the second largest copper producer in sub-Saharan Africa, insists that its debt pile stands at $8.7 billion - but there have been suggestions it could be double that.The allegations have raised fears among Zambians and foreign investors for whom the memory of Mozambique's hidden debt fiasco is still fresh in the mind.In 2016, international donors froze fiscal support to Mozambique after it emerged that the country had taken $2 billion of "off-book" loans to finance maritime vessels and military equipment.That move plunged the impoverished country into serious economic and financial crisis.Maputo has since suspended repayments to creditors and is seeking to renegotiate its debt mountain.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookJapanese finance giant Nomura recently questioned the size of Zambia's public debt, suggesting the government is concealing a substantial amount of short-term external debt.In a note last week, the director of the political risk advisory firm EXX Africa also warned of "mounting evidence of undisclosed loans" and said that Zambia had miscalculated its total debt."Based on documentary evidence, we assess that external debt could be as high as $15.6 billion, while local debt seems almost incalculable given the opacity in lending to state-owned entities from local banks," wrote Robert Besseling.'Hiding the figures'? But Zambian authorities have brushed aside the growing concerns over the actual size of its external public debt, which has been putting pressure on the value of government bonds.Former finance minister Felix Mutati last year told parliament that foreign debt stood at $17.2 billion - but retracted the figure days later saying he had misstated it and insisted that the debt burden was just $7.2 billion.But the damage had already been done.Chishimba Kambwili, a ruling party lawmaker and former minister accused his own government of cooking the books."They have been hiding the figures," he told AFP, suggesting the debt could be as high as $23 billion."Let them not pretend that all is well. That is unacceptable."Former vice president and now opposition politician Nevers Mumba is also convinced there is an official cover-up."Our investigations revealed that the foreign debt was at $16.6 billion," Mumba told AFP. "The numbers that they are giving are not correct and they have failed to disprove our figures."But the government has doggedly stuck to the most recently published figures, vehemently denying that it has concealed any debt.Finance Minister Margaret Mwanakatwe "has repeatedly said that our debt is at $8.7 billion and let people not speculate," Treasury spokesperson Chileshe Kandeta said."If someone has evidence that it is not $8.7 billion, let them supply the evidence."'Undermining its macroeconomic stability' Zambia had been expecting to secure a $1.3 billion loan from the International Monetary Fund (IMF) this year, but the Washington-based lender is unhappy with the country's borrowing plans.The IMF's representative to Zambia, Boileau Loko, said officials "continue to compromise the country's debt sustainability and risk undermining its macroeconomic stability".Allegations that the country has not openly disclosed its external debt burden have undermined investor confidence and driven up borrowing cots, according to London-based BMI Research."With the spectre of 'hidden debt' a continued concern for regional investors after the 2016 hidden debt crisis in Mozambique, and in the continued absence of a deal with the IMF, we believe borrowing costs are likely to remain elevated," said BMI in a note on Friday.It added that the situation heightened the risk of Zambia needing to extend the repayment period for its loans.After posting average annual growth rates of more than 10 percent in the 2000s, growth has slowed significantly in recent years due to falling copper prices.But Mwanakatwe, the finance minister, has forecast growth will accelerate to four percent in 2018 as copper prices begin to recover.