Safety regulators have quietly placed two extra conditions on construction of TransCanada Corp.'s Keystone XL oil pipeline after learning of potentially dangerous construction defects involving the southern leg of the Canada-to-Texas project.

The defects — high rates of bad welds, dented pipe and damaged pipeline coating — have been fixed. But the federal Pipeline and Hazardous Materials Safety Administration wants to make sure similar problems don't occur during construction of the pipeline's controversial northern segment, which is on hold pending a decision by the Obama administration.

One condition requires TransCanada to hire a third-party contractor chosen by the pipeline safety agency to monitor the construction and make reports to the safety administration on whether the work is sound.