The statement quoted Akihiro Seita, the director of the agency's health program, as saying that the sudden upswing was unprecedented in the Middle East.
"When the 2013 results from Gaza were first uncovered, UNRWA was alarmed by the apparent increase. So we worked with external independent research groups to examine the data to ensure the increase could be confirmed," he said. "That is why it took us so long to release these latest figures."
He suggested that Israel's blockade of the coastal strip, where close to 45 percent of the population is under 14 years old, could be a contributing factor.
"It is hard to know the exact causes behind the increase in both neonatal and infant mortality rates, but I fear it is part of a wider trend," he said. "We are very concerned about the impact of the long-term blockade — on health facilities, supplies of medicines and bringing equipment in to Gaza."
Israel imposed its blockade on the Gaza Strip in 2006 after Hamas captured an Israeli soldier. Israel tightened controls a year later after Hamas consolidated its rule in the strip.
A 50-day war between Israel and Palestinian armed groups in July and August of 2014 killed about 2,200 Palestinians (mostly civilians) and 73 on the Israeli side (66 soldiers, six Israeli civilians and one Thai national) and destroyed or damaged tens of thousands of homes in the impoverished coastal territory.
Among the Gaza dead were more than 500 children.