Former Fed Chairman Alan Greenspan said Thursday that monetary policy has reached the outward bounds of its effectiveness without another round of quantitative easing.

"Monetary policy … has done everything it can unless you want to put additional QEs on. They're not helping that much in the sense that ultimately determines whether or not you're getting an effect from the QEs" beyond increasing price-to-earnings ratios in the stock market, he said during an interview on CNBC's "Squawk Alley."

"There's no real evidence that we're getting an impact on lending and on the economy picking up," he said.

Greenspan said he disagreed with International Monetary Fund Managing Director Christine Lagarde that negative interest rates create a net positive impact. Lagarde offered the assessment earlier on "Squawk on the Street."

Japanese and European policymakers have pushed some key rates into negative territory.