A mysterious Russian satellite is raising concerns after a Department of State official said it has been displaying 'very usual behavior' and there is no way for the US to verify what it is.

Dr Yleem Poblete, assistant secretary of state for arms control, verification and compliance, said Tuesday that the satellite was launched in October and 'its behavior on orbit was inconsistent with anything seen before'.

'We don't know for certain what it is and there is no way to verify it,' she said in a speech at a UN Conference on Disarmament, a forum established to negotiate arms control and disarmament agreements.

'But Russian intentions with respect to this satellite are unclear and are obviously a very troubling development.'

A Russian satellite (not pictured) is raising concerns after a Department of Defense official said the satellite is displaying 'very unusual behavior'

According to Poblete, the US has not been able to verify whether the satellite could possibly be a space weapon and is raising concerns about what the Russian government had deployed.

'Now, I can tell you that our Russian colleagues will deny that its systems are meant to be hostile. The Russian Ministry of Defense has put out a press release stating these are simply inspector satellites,' she continued.

'So the question before this body is: How do we verify what countries say their spacecraft are doing? What would be enough information to prove what the purpose of an object is?'

Russia responded to Poblete's claims saying the satellite is a 'space apparatus inspector' and has no hostile intent.

Alexander Deynko, a senior Russian diplomat, told the Reuters news agency that the comments were 'the same unfounded, slanderous accusations based on suspicions, on suppositions and so on'.

The concerns were raised days after president Donald Trump announced plans for a sixth military branch called Space Force, which will focus on guarding the US against any hostile action in space.

Russia has denied that the satellite is of hostile intent. The concerns were raised days after president Donald Trump (left with Russian president Vladimir Putin on the right) announced plans for a Space Force military branch

During a trip to Brazil on Tuesday, Defense Secretary Jim Mattis defended the plan for a Space Force telling military officers and civilians at a Brazilian war college that it is necessary to protect American satellites from being targeted by Chinese and Russian attack weapons.

Mattis recalled China's use of a ground-based missile to destroy one of its own nonfunctional weather satellites in January 2007, which he suggested was a calculated demonstration to the United States of Chinese capabilities.

'We understand the message that China was sending - that they could take out a satellite in space,' he said. 'Since then our intelligence services have watched other nations, including Russia, develop a space attack capability.'

Mattis said the US cannot ignore potential threats to satellites that are crucial to communication, navigation, weather information and other underpinnings of modern life.

'So this is a reality,' he said. 'We are not initiating this. We are saying we will be able to defend our satellites in space. At the same time, if someone is going to try to engage in space with military means, we will not stand idly by. We don't intend to militarize space. However, we will defend ourselves in space if necessary.'