"Whether it's 3 months or 6 months or 18 months, it is soon," Dunford responded. "We ought to conduct ourselves as though it's just a matter of time and a matter of very short time before North Korea has that capability."
Dunford also agreed with Inhofe that it's especially difficult to get intelligence on North Korea even with the U.S. military's aerial snooping capabilities.
"The North Koreans over time have buried much of their capability underground, which creates new challenges," said the general. "There's also some specific weather challenges in North Korea that limits our collection at various periods of time."
Besides satellites, the U.S. uses reconnaissance aircraft to monitor the North Koreans along with other means.
However, Dunford indicated that the U.S. military has faced other challenges over time in constantly tracking North Korea because of needs it sometimes has in other hotspots around the globe.
"Part of it also has been the competing demand for a limited amount of intelligence, surveillance and reconnaissance," said the general. "Certainly over the last 18 months, we have increased our collection against North Korea. But for a long period of time we had decreased our collection against North Korea."
Dunford also was asked about North Korea perhaps sharing its missile and other military technology with Iran.
"We have looked at the nexus quite a bit," said Dunford. "I'm not sure we've seen any transfer of nuclear technology, but we certainly have seen missile technology and a wide range of other weapon systems that they have exported or expertise that they have exported outside of North Korea."
The Obama-era Iran nuclear deal and Iran's increased military presence in other countries also came up during the hearing. There's a congressional requirement that the administration provide an assessment to Congress of Iran's compliance with the so-called Joint Comprehensive Plan of Action (JCPOA) every 90 days.
Sen. Mazie Hirono (D-Hawaii) asked the general whether it was in America's national security interest to keep the JCPOA.
Dunford said it was the intelligence community's assessment that Iran is "in compliance right now. Therefore, we should focus on addressing the other challenges — the missile threat they pose, the maritime threat they pose, the support of proxies, terrorists, and the cyber threat they pose."
Even so, the general said he doesn't plan to publicly share the advice he will give to the president on the Iran nuclear deal until Trump has announced a decision.
The Trump administration has certified Iran's compliance twice under the law, and the next deadline to provide an assessment is Oct. 15. Trump, who had been sharply critical of the Iran nuclear deal, has indicated he's made his decision on whether Iran is in compliance but hasn't shared it.
In his written testimony to the Senate panel, Dunford said Iran hasn't changed its "malign" activities since the 2015 Iran nuclear deal went into effect. However, Sen. Tom Cotton (R-Arkansas) asked the general if Iran had "increased the pace or scope of their malign activities."
"I think you can argue that they (Iran) have certainly in Syria," Dunford responded. "It's been relatively consistent in Yemen with regard to their support for the Houthis, and clearly their support for Lebanese Hezbollah has been at a high level for some period of time. And you can argue over the last few months, whether it's related to JCPOA or not, that Iranian activity inside Iraq has certainly increased as they look to the end game in Iraq."
Finally, Dunford said if U.S. forces are threatened by Iran "they are both postured and capable of effectively responding."